Hello readers, Welcome to my crash course on Nasus
This is the first build I've made, and I freely admit that most of this guide was formed after following Hyfe's Guide to Competitive Nasus. I don't claim that this guide is better, which in all likelihood it isn't (please don't down vote me for that reason), but I wanted to throw it into the pool mostly because I've always wanted to post a build.
The other part of why I wrote this is because every time there is a Nasus free week I suffer physical pain from the amount of terrible MAD DEEPS Nasus' I see people build.
I'd love to hear how I can make this guide better, so please leave a comment!
But enough about me, what about Nasus?

Select Nasus if you want:
-Strong farming
-strong lane presence
-good last hit training
-Good amounts of health and survivability
-Strong burst damage
-An Ulti that will draw attention away from the carries
-to be able to push easily and eat towers for breakfast
-A smooth sexy voice that will help you keep your cool in all but the most rage enducing circumstances

Don't select Nasus if you cannot deal with:
-not having any ranged capability
-dependence on early game farming
-Only one big Damage skill
-Being kited
-Needing to co-ordinate with your team

This is all good to know, but it doesn't answer the question, that being: WHO IS NASUS?
I won't go into lore, you can read that in your own time.
Nasus is a very versatile Tanky-DPS champion. He can push with spirit fire, Deal heavy damage to anything on the field with Siphoning Strike, Shut down an auto-attack carry ( Ashe, Tristana, etc) with Wither, and make any team go wide-eyed in terror when you use Fury of the Sands. however, his lack of range means that champions with heavy range can kite him all day long, which is truly his one weakness, wither can only do so much to stop it.

hey hey. haven't updated this in forever, but man, Nasus buff about a month ago and my god was it AMAZING!
Better starting lifesteal makes him so much harder to push off lane it's not even funny, and the instant spirit fire with initial burst changes our skill order for emphasis on spirit fire after we max Siphoning strike, also makes the E>W>Q combo a lot easier to pull off for MUCH more damage! Thanks riot!

Innate: Soul Eater
Start the game with 15% life steal. sweet. get 17% at level 6. better. 20% at 11? great! and all without spending a single gold. Gives you sustainability in laning, letting you recover from harassment faster. also a reason some people like to jungle nasus.

Q: Siphoning Strike
Nasus' signature skill. it deals damage, plain and simple. And the more you kill with it the better it gets! This skill is one of the reasons I recommend everyone plays nasus at some point during your LoL career, because it teaches the value and timing of last hitting. Getting a farmed Siphoning Strike can make or break your game as nasus, so even if it means a little harassment wade in and smack that minion for the kill. Siphoning strike deals its extra damage to turrets as well, so when you're trying to push down a tower mash Q like it's going out of style. an important thing to note about siphoning strike is that using it resets the auto-attack timer, meaning that if you use it in the moment right after nasus strikes an enemy with his normal attack he will skip the "recovery" time from the attack and immediately attack again with siphoning strike at least 60% of your mana during lane phase should go into this skill

W: Wither
I am of the opinion wither is the greatest single target slow in the game. It's the slow that keeps on slowing. Use it on the auto-attacking carry in a team fight to destroy their damage output, or use it on a fleeing enemy to practically make them stop dead. NEVER use wither on Master Yi as Highlander will immediately cancel both the move speed and the attack speed debuff, the same applies if he is already using highlander It is also important to be careful when using wither on Garen, as Judgment will have a similar effect. if possible wait until moves that decrease slow effectiveness or durations are over on your target. This is a very costly skill mana-wise, if you're going to use it during the laning phase, make the most of it.

E: Spirit Fire
Spirit Fire is an amazing skill post buff. upgraded from a simple farming utility into a slight nuke as well, this will make a difference in the lane and the teamfight. It deals enough AoE damage to leave a mark on minions while reducing their armor so your own minions can get through them faster, it also allows you to Last hit with siphoning strike easier. This skill is now much better in a teamfight for the initial burst, which, while still weak, is better than before, and the armor debuff will have the AD carries singing your praise. A mistake I see a lot of new nasus' make is spamming spirit fire early game. While sometimes you may not have a choice, like if you have the solo lane due to a jungler, it's important to try to use this skill sparingly.

R: Fury of the Sands
Hereafter referred to as Fury. Nasus' ultimate is what you will be using to attract attention during teamfights, because, lets face it, Empowered Nasus is one of the scariest things you can encounter in League of Legends. Not only do you get a nice chunk of health back (note that having a grievous wound does not affect the health gained from Fury), any enemies near you slowly take damage equal to a percent of their max health from a sandstorm that rages around our champion, making this a Must for fighting HP stacking characters. But Wait! There's More! Damage dealt by the sandstorm boosts your attack damage! so even MORE DAMAGE! That said, Nasus' Ulti is very easy to waste, and requires a good sense of timing. Use it too early in a team fight and the enemy team will run, use it too late and your team members will already be dead, meaning you get focused down. the only way to learn the right timing is through experience and intuition, but once you get it down you'll be a force to be feared.

for summoner spells, I like:
Teleport
Sometimes laning phase goes badly and you take a lot of harass, and need to B or give up a kill. sometimes you get ganked by the enemy mid or their jungle, sometimes you just have farmed up a ridiculous amount of gold because you got to freefarm. sometimes you can save a tower, sometimes you can ensure a tower gets destroyed. sometimes you can set up a gank. point here is, the map presence granted by teleport is amazing.
Exhaust
Exhaust increases your utility and your danger by a TON. not only is it a gank tool, it allows you to shut down a Second AD champion in teamfights, and can help you to escape in a pinch.

Other acceptable spells:
Ghost
Well as great as the movespeed from ghost is, and as fun as it is to zip around the map after you have your tri-force and ghost, I like the presence granted from teleport much more
Cleanse
If there's one thing that makes a team angry it's when a champion simply doesn't die, and with Mercury's Treads, Banshee's Veil and cleanse, you will laugh at their CC and run to safety. also useful when a fleeing champion tries to CC you as a last ditch effort to save them self, Cleanse it and claim your victim.

Flash
Flash is a favorite spell of mine, for carries. if you need more or less distance on an enemy ghost and one of your slows will do the job. yes this means no wall hax, but it's an acceptable loss.

Heal
I find that heal is either over rated or under rated. it's strictly average. never the best choice, never the worst. I will admit that it's startling how many players don't check their enemies summoner spells, so you might be able to pull first blood for your team by surprising the enemy with a last second heal. not bad, not good. I don't think it's worth the utility that ghost and exhaust provide.

Clairvoyance
Second to map presence is map awareness, and Clairvoyance provides that with gusto. Use it to check your lane bushes early game, scout the enemy jungler, check baron or dragon, and more. That being said, it's a weak choice on Nasus. This is better left to straight support champs.

Ignite
The early game bane of carries everywhere. A DoT that has great damage early-mid game, but quickly loses impact. Good for use on regen and health steal reliant characters like Mundo or an enemy Nasus. However kills are not your top priority, you should be letting the carry grab the kill whenever it's reasonable, so I advise you leave this one at home.

Clarity
Ideally with the MP/5 runes, perseverance, and meditation Mana won't be a big concern, and you'll also be grabbing blue buff if no one else wants it (ASK BEFORE YOU TAKE. EVERY TIME. I DON'T CARE IF YOUR TEAM IS RENEKTON GAREN KATARINA AND MUNDO, ASK ANYWAY.) however for those of you who aren't level 30 this could be an acceptable substitute for one of your spells

Fortify
It has it's uses, saving a tower, or killing a champ that gets taunted or otherwise knocked into tower range without minion cover, but I still don't like it because most of the time all you're achieving is delaying the inevitable and messing up your teammates last hits. and the minion damage doesn't make THAT much of a difference.

summoner spells to avoid:
Rally
I rarely see rally used, and for whats it's worth, a skillfully placed rally is pretty awesome, but it's DEFINITELY not something I'd stick on nasus.


Smite
While nasus CAN jungle, it denies a lane of a very powerful laning phase champion, so I wouldn't recommend it. since you're not in the jungle, stay away from smite.

Revive
do I really need to explain?

Runes
I picked Greater mark of desolation and greater quintessence of desolation because as an off-tank you need to have some threat. with this armor pen during the first team fight you'll be able hit the carry for a sizable chunk of their health, and thus establish yourself as something that shouldn't be left alone for future fights, but that's OK because you've got plenty of survivability to take the brunt of the damage that slips past the tank

Greater Seal of Scaling Mana Regeneration eliminates your need for any MP/5 items, which is good because there aren't really any good tank items that provide it. The tiny amount it gives you early game is enough to support your siphoning strike last hitting, while the larger boost late game should let you stay out as long as you want without mana becoming a big issue.

then we have Greater Glyph of Scaling Magic Resist to give you some survivability throughout the game. An alternative to this is Greater Glyph of Cooldown Reduction, which will allow you to last hit with your q a lot more early game, but I feel that the CDR causes you to run out of mana from last hitting with Siphoning Strike as well as pushing the enemy tower, which is just begging for a gank. The 9% from masteries is a nice sweet spot, allowing you to go faster than normal but still keep the minion waves from going too far either way.

Mastery wise I went with 9-0-21
HUH? why 21 in utility?
We could go with 21-0-9 but your goal isn't MAD DEEPS (lots of damage in the common tongue).
we could take 9-21-0 but honestly the only defensive masteries that have a lasting impact on the game are Strength of Spirit , Evasion , Nimbleness , Ardor , and Tenacity. yes, I am saying that the 48 health you spent 4 mastery points on is useless in the long run. so is that 6 Magic resist and armor. on the other hand, movespeed, summoner spell cdr, regular cdr, mana regen, and health regen will be useful all game, leaving you with 4 "useless" points (from increased experience) late game as opposed to 10 (from armor, Magic resist, and health)
My Philosophy is to have as few "crutch" runes and masteries as possible. even the often praised (and very expensive) Greater Quintessence of Health quickly become vestigial. Yes early game performance is important, but smart play and nasus' natural longevity in lane will allow you to be a more powerful presence in mid and late game at the price of some early game durability.

My usual skill order is placed at the top of the Page, with priority placed on siphoning strike, however there are a variety of different situations that can arise, and nasus can level his skills differently for each one. so PLEASE use your best judgment and skill according to the situation. Here are some example situations and how you should place skill priorities for each
what are skill priorities? basically, whenever available, put a point into skill listed as 1. if 1 is unavailable, do skill 2, if both 1 and 2 can't be leveled put the point into 3, etc.

Please note, I highly recommend having one point in all three non-ultimate skills at level 3

Basic laning: it's a 2v2, you have no jungler
Skill priority:
1. Fury of the Sands
2. Siphoning Strike
3. Spirit Fire
4. Wither

this is your basic farming skill set. you want siphoning damage first, slow for when/if mid comes to gank, and good AoE damage for pushing and harassing opponents. if you're duo laning, make sure that the carry goes with the support, and not you, and DO share your minions with your laning partner. yes your Creeps Slain score is important, but no need to hog, yes farming Q is important, no don't gimp your lane mate because of it. Do make sure to smack a champion with Siphoning Strike if they're ever stupid enough to approach you. one thing you do NOT want to do is push to the enemy tower. It's tempting, I know, but the risk of getting ganked because mid missed their MIA call or the enemy jungler coming out of no where is real, and caution is necessary. Keeping the creeps more or less in the middle of the map is good, but actually letting your opponent slightly push is best, because it makes your teams ganks much easier.

Bad laning: 2v2, your partner is feeding/disconnected/rage quit, you have no jungler
1. Fury of the Sands
2. Spirit Fire
3. Siphoning Strike
4. Wither
This will allow you to tower hug and farm your Q at your tower, hopefully the enemies will be too scared to dive you since hopefully you aren't following your idiot teammate out to his death. In this situation, as hard as it may be, don't get angry at a feeder. You'll have to act as if your in a solo lane as soon as you realize your partner is feeding. Start playing Defensively, let the minions push to your tower and kill them there. when trying to Last hit minions under your tower, I find that allowing the tower to hit the melee minions twice before using Siphoning Strike is the best way to farm them, smack Caster minions after they get hit once. bottom line in this situation is self preservation. you don't want to offer up more gold to your foes, so don't take risks.

fun laning: 2v2, you have a jungler, a stealth character, or a similar ganking party member
1. Fury of the Sands
2. Wither
3. Siphoning Strike
4. Spirit Fire
I call this Fun laning because whats more fun than letting foolish enemies push to far over and over again only to find their death waiting each and every time? in these situations you really want to let the enemy push, but be very zealous about defending the tower. if you see the Ganker mark a target, pop your ghost and wither/exhaust your victim and commence beating the **** out of it. You may need to use your other slow on the remaining enemy champion if you think one of your allies is going to die. If you notice the ganker coming to your lane, you can also mark the target if you notice he hasn't been. your pumped wither will mean a guaranteed kill whenever your Gank comes around, and your siphoning damage won't suffer much

solo lane: 1v2, you have a jungler
1. Fury of the Sands
2. Spirit Fire
3. Siphoning Strike
4. Wither
Try to have a point in wither by level 4. Nasus is a great candidate for solo lane, but don't be reckless, it's still 1v2, and your lifesteal can only heal you up so much. when playing solo, the enemy minions should be in tower range as often as possible, and with them, the enemy champions, so ganking is easy. your siphoning damage will definitely not be crazy, but in that situation it's OK, because the Experience bonus still leaves you ahead of the competition.

WHY?: you are mid. this shouldn't happen but I've had to deal with it before.
1. Fury of the Sands
2. Spirit Fire
3. Siphoning Strike
4. Wither
there's always a funny story behind nasus taking mid, but you should treat it the same as situation 2, because there's going to be a lot of damage focused on you, and you can't always count on a gank. just play it cool, keep your distance, and call for ganks, but don't be annoying about it.

Lastly, during laning phase I've seen many people recall at half, 3/4, or even full health because they have enough money for the next item in their build. this is bad. Gold, Experience, and Siphoning Strike damage are much more important than grabbing an item. There have been situations in which I haven't recalled until 25 minutes into the game and had enough to straight buy a Trinity Force. The game didn't last much longer. That is what I challenge players using this guide to do: Try to stay in lane until you have enough for your trinity force, and if you manage to get enough, Congratulate yourself by buying it as your first item after Doran's Shield. the movespeed gain won't be as good, be from that point on farming and killing enemy champs will be a lot easier. of course that doesn't mean be stupid about it. there's no failure in not completing the challenge I set, so if it means the early trinity force or your life, pick your life.

Pop Quiz! out of the following, what is Nasus' top priority?

Farming siphoning strike damage
Pushing
team fights
all of the above
none of the above

time's up!
those of you who answered with "all of the Above" or "none of the above", please come to the front of the class for a gold star. unless you cheated.

how can nasus' top priority be all of those and none of those?
like I said at the beginning of the guide, Nasus has many uses, and he performs well in any of them. Nasus' top priority is whatever the party needs most at the time. If you're winning team fights but can't push down a tower before the enemy team recovers than split off from the group and demolish a Turret. while the rest of your team occupies their attention. if you get ganked while doing this and your team manages to push down a tower, you've still served your purpose, but don't make a habit of dieing for a turret.

If your tank can't Hold all the damage in team fights than Stick with the group and lend your crazy burst to the team in a melee. Remeber to use Spirit Fire to melt through the enemy teams armor, and make sure to mark the your teams target if no one else does.

In general, once laning is over, you need to identify what area's your team isn't performing well in and apply yourself to strengthen that point. here's a quick checklist
your towers vs their towers
your teams kills vs. their teams kills
Team fight performance
Personal performance

I will admit, the idea of having 400 bonus siphoning strike damage and being able to practically one shot the enemy AP carry on a crit is a pleasant one, but All the siphoning damage in the universe won't save you if a Yi backdoors you or your team gets destroyed in a team fight, so once laning phase is over, Farming your q is the last thing on your to do list. Start looking for what you can do to improve the game, and if everything seems to be going smoothly, sure you can go back to farming your siphoning strike, but make sure to tell your team and Watch for pings.

Nasus is a very special character. The way his Siphoning Strike gains damage as the game goes on means that a skilled Nasus can build primarily tank gear and STILL get respectable damage.
for that reason, we start the game with Doran's Shield. The armor, health and health regen combined with nasus' passive lifesteal makes you tough to kill early game, and it means that nothing short of unrelenting concentrated harassment will take you off the creep line.

as I've said already, Movement speed is god, so the first thing you should buy when you go back to the fountain is Mercury's Treads. Movement speed, Magic Resist, and Tenacity in one fantastic package.

third priority item is Sheen so you can start winning. lame jokes aside, Any Nasus player worth his salt can tell you that Sheen is the one must have item on Nasus, it means your big damage skill hits a LOT harder, allowing your Siphoning Strike damage to snowball faster.

by this point things should start moving into ganking and team fight phases

Next on my list is Aegis of the Legion. In my opinion, Aegis of the legion is the off-tank item, giving you more survivability, extending some protection to teammates, with even a little bit of damage on top. whats not to love?

We get even tankier from here, folks. Pick a up a Banshee's Veil and you're ready for nearly any encounter. This solves 3 problems: you have a respectable pool of mana, so conserving it isn't as big of a deal. you have enough MR to not get insta-gibbed by an AP carry during a fight. And the spell shield allows you to initiate a team fight if the usual initiator is indisposed. you've also accumulated a pretty hefty sum of health once you have this, so start watching out for Death Fire Grasps or Madred's Bloodrazor, as these will tear you apart.

This is as tanky as we need though, so build your Sheen into a Trinity Force for damage, movespeed, mana, crit chance, health, an improved sheen effect, and a chance to slow on hit. this is when you can start eating things alive while Empowered from Fury of the Sands

The last item is situational. I like to put a little more damage on but sometimes that isn't an option. here are all the items I recommend:

In 75% of all games I end up going with an Atma's Impaler because it balances out your Armor and Magic Resist, and will bump your Attack damage up by a lot, and it only gets bigger when you pop Fury of the Sands. high AD+Siphoning strike+critical hit can usualy hit a squishy for half of their health, so the crit chance is welcome as well.

Infinity Edge is what I normally go with if we have a good tank but need more damage. the hefty damage boost and crit chance/damage leads to Parrrley like crits.

If the other team is AP centric and your Main tank is struggling Force of Nature can be worth your time, primarily because of the massive Magic resist boost it grants, you also have enough health to capitalize on its unique passive, and movement speed is always welcome

on the flipside, if the team is AD centric and your tank can't handle it Thornmail, Frozen Heart, and Randuin's Omen provide fantastic Armor boosts while actively destroying the hopes and dreams of auto-attackers everywhere. If you end up going with Randuin's Rememember to make use of its Active ability. It is fantastic for teamfights.

Zeke's Harbinger adds another fun aura to the mix, lots of attack speed and reduced armor on enemy champs, as well as some lifesteal can be entertaining. the reason I recommend this and not Bloodthirster is because Starks benefits the entire team.

Items I don't like but you can try if you REALLY want to
Bloodthirster: stacking lifesteal on nasus sounds hilarious, but I've found it unimpressive.

Sunfire Aegis: while the armor is nice, you really don't need more health at this point, and you'll get more use out of Atma's Impaler, especially since the sunfire nerf of 1.0.118

Shurelya's Battlesong: CDR, HP/5 and MP/5 are cool, but like I said with sunfire cape, you don't need more health at this point. I will say that the active is wonderful to have around.

As a last note in items section: When you have all 6 slots of your inventory filled, you can still purchase elixers, they will be automatically consumed. so if the game is dragging on and you finish your build, start chugging elixers. yes, all of them. Alternatively you could save up and replace your Doran's Shield with its big brother Randuin's Omen if you haven't already bought it.

While I have expressed my distaste for DPS nasus, after having played a few games with a D/C'd or leaver AD carry, I feel that sometimes the need arises for some sustained damage, and nasus can step up to the plate. this section won't be as detailed as the last one because it's really for those oh@#$% games when your missing a big damage dealer
you still start with Dorans shield, but get your Sheen before Mercury's Treads, then finish out your Trinity Force. instead of an Aegis of the Legion you'll get an Infinity Edge, still get a Banshee's Veil for some survivability, and then Bloodthirster pushing your damage and lifesteal to hilarious levels.

COMMON SENSE
unfortunately, it isn't so common anymore, some might even say it's a freaking superpower, but please keep things realistic. don't push the inner tower when the entire enemy team is MIA. don't try to 1v3 because you got a double or even a triple kill earlier. don't chase the enemy carry across the map. be prepared for the worst. Expect to here that DEMACIAAA every time you walk into a bush. Expect Rammus to Powerball out of nowhere when you've got the enemy carry cornered. Expect a Warwick to Infinite Duress your carry as soon as the team fight starts. Runes, Masteries, and items can do a lot for you, but if you play like an idiot you WILL lose.

That's all from me. I hope you learned something from this guide, and I hope your Nasus game improves!