Hello fellow summoners,

this is a guide to my way of playing Nasus. I have not yet found another guide on this site that I can completely support and decided to give it a try myself.
The item-, rune- and masterybuilds I am using here are an outline of what I use ingame and I will do my best to provide useful alternatives for different situations and playstyles. I will also try to explain my choices over others.
Hopefully this guide will help some of you on your way to playing a truly awesome champion that I find is more often left aside.
This guide focuses on players that are level 30 and have a more or less complete runepage for this as explained later on. Player of lower levels might find this useful but might find it hard to use since the quintessences are a crucial part of the build.

Have fun reading and testing this guide. I will be happy to get some feedback and more ideas.

Saccharom

A Nasus playing this way is mainly focusing on playing solo top, tanking (a lot), adjusting the enemy teams health bars a bit and walking around letting all those armors you bought shine.
Nasus, as any other champion does have his Pros and Cons, some of them are:

Pros
- Awesome tank with good damage
- Good single target slow with Wither
- Can carry late game
- Excellent sustain
- Great tower pushing with his Siphoning Strike

Cons
- No real ranged damage for harass
- Relatively weak early game
- Dependent on farm

The point that he excels against certain champions and completely sucks against others is neither a pro or con I believe. Nasus does an awesome job on top lane solo but can be countered by certain champions. Mainly these are ones that have great harass and/or range ( Gangplank, Irelia) while holding up with his sustain. In general I find that since being counterpicked is a thing of bad luck (if you play Blind Pick) and that those champions that have good chances against him can still be dealt with.

Obviously the masteries focus mainly on defense. I believe that the details are more or less personal preference. What I find crucial is putting some points in the movement speed as this helps you dodge skill shot abilities, get to teamfights and back to lane faster and it just isn't fun being a brick.
The point in reducing turret armor is also a nice thing as the damage of your Siphoning Strike does apply to tower hits and this can let you quickly take down towers that enemies thought safe for another few minutes.
These are the only points I find are really important to speak of. The others speak for themselves and some might just be personal preference over others.

First off, I found that Mobafire seems to have an incomplete Database on the runes and is missing the Greater Glyph of Magic Resist. I have put in the MR/lvl glyphs as a placeholder, but I tend to use the flat MR.
Now to the runes. Again, this is what I use and have experienced to serve me well.

Greater Seal of Armor and Greater Glyph of Magic Resist don't need much explaining. armor helps you a lot early game and magic resistance is good all through the game. I use flat MR as it gives you +12 MR from level 1, while MR/lvl would give you +1.35 at level 1 and +24.3 at level 18. The bonus they give you late game seems to be marginal looking at your overall magic resistance with the item build. I found the advantage flat MR gives you early on lane far more useful than those extra +12.3 MR at level 18. If you find MR/lvl to suit you more, I wont keep you from using them.

Greater Mark of Desolation can give you an awesome boost to the applied damage of your Siphoning Strike. I don't find any of the other marks to equal these in usefulness. I have seen people use Greater Mark of Critical Chance for critical damage on their Siphoning Strike but don't like the thought of this as it makes you depend on chance rather than fixed values.

Greater Quintessence of Vigor is one of the keys to this guide and build. Again I use the flat bonus over the stacked for maximum efficiency early game on your lane. With three of these quintessences you will be able to regain health at a considerable rate that will let you stay on lane for a long time into the game. When building your runeset for Nasus these are the ones you want to grab first.


Alternatives
You might want to use runes that give a stacked per level bonus instead of these. I don't recommend that though as this runeset is focused on improving Nasus's early weaknesses. Later in game you will rely heavily on your items and the bonuses from the runes wont make much of a difference here. All of the other runes don't really fit into this build.
Another option some choose is taking Greater Quintessence of Life Steal. As this can take the role of holding your sustain early in game too it relies on you landing a lot of hits with your Siphoning Strike. If you happen to be heavily zoned by a ranged champion for example you wont get to that. Using the health regeneration quintessences makes you regenerate health when you are towerhugging as well as when you are allowed to play more aggressively. Some may find the lifesteal to suit them more though.

The choice of Summoner Spells in this guide really is a personal preference. I find Heal and Ghost to be amazing spells that have saved me so many times and secured me many kills.

Ghost can be used best to run back to your tower when ganked, chase down dying enemies to give them one last Siphoning Strike or simply get to your lane or a teamfight faster.

Heal is amazing. It can do amazing things. I know it is often frowned upon as "noob heal" etc. but come on, if it is so good then just use it yourselves will you. I use heal mainly as a means of either saving myself, healing the team in a fight or securing kills. The first two are obvious, the latter works if for example you are alone on lane, your counterlaner is fairly low in health and the enemy jungler comes in for a gank. It sometimes works out that you hit the jungler down a bit, drag them both under your tower and pop your Heal when they are diving into tower fire. More often than not this can give you a sweet kill, sometimes you even get both if they are greedy.

Other viable choices for your Summoner Spells can be a variety of spells. Flash can be used instead of Ghost if you are more comfortable with that. Teleport also comes in handy sometimes. If you like you can even use Ignite or Exhaust for use in teamfights. If picking any of these I personally would still stick with Heal as primary spell since its uses are just great. Did I already say that Heal is amazing?
The other spells really should not be taken with Nasus as they just don't help you very much.

The skill sequence is very straight forward.

Siphoning Strike
Since the buff that Siphoning Strike received some time ago, putting the bonus damage you get for a killing blow from +2 to +3, this is what you will max first. Imagine someone coming up to you with a box. In this box was a button. He would tell you that if you press that button you will get 100 dollars (or euros or pounds or what ever). You wouldn't press the button once and go buy yourself a new pair of shoes. You would hit that button as often as humanly possible before buying yourself that new pair of shoes.
Well let me tell you, Siphoning Strike works pretty much like that. Everytime the enemy laner lets you last hit a minion he will be very very sorry for it later. So you best get Siphoning Strike to level 5 for that extra cooldown and you'll be stacking damage like crazy.
Obviously you concentrate on last hitting minions with this during laning phase, later on you can grab last hits whenever you have the chance without making it a priority.
When fighting enemy players you will want to spam this as soon as it is off cooldown. The damage you will inflict is simply astonishing.

Wither
This ability is a great slow. It will help you chase others down, prevent others chasing you and help your jungler with his gank. On level 5 it will slow the target for 5 seconds bringing it down to 5% of its movement speed at the end of the slow period. If the target has a tenacity item such as Mercury's Treads or Cloak and Dagger the duration of the slow will be decreased but he will still be slowed down to 5% of movement speed at the end of it. To face it 5% is pretty much standing still and can really ruin someones day.
On lane you should go easy on using Wither as it will drain your mana if used too often. Take it for escaping ganks, initiating a gank by slowing the target just before the jungler comes out of his cover or securing a kill for you or your jungler.
In fights you can pretty much use this at your own leisure as fights shouldn't last long enough for you to run out of mana and having someone slowed can always secure a kill for your team. Note that slowing an enemy will get you an assist so if you arrive late to a gank or fight that your team was winning anyway you might want to slow a dying target simply to get some gold for an assist without stealing the kill.

Spirit Fire
This spell is both over- and underestimated at the same time. It has three direct effects. The first is the damage dealt to targets that are standing in the area of effect upon casting Spirit Fire, the second is the damage over time that is dealt to enemies standing on the area of effect during the duration of the spell and the third is the armor debuff that enemies on the area of effect receive. The damage effects are more or less to be ignored against champions but are noticeable against minions. The armor debuff though is pretty useless against minions but can have huge effects when placed well in a teamfight.
While you are on your lane you will want to use this to either thin out a large minionwave that is about to push your tower or simply give the enemy champion that armor debuff when your jungler closes in for a gank. When maxed Spirit Fire deals enough damage to take out minions that remain standing in the area of effect for the duration and can be used for hit an run farming minion waves. It can also be used as a means of crowd control as most players seem to start running out of it in panic and avoiding the area as long as its active. As you learn how your enemy is playing and whether he responds to this or not can rule this option out or make it a fun thing to use for zoning them.
If you are in a teamfight Spirit Fire can turn the tide if placed right. It will hurt the entire enemy team but especially the tank ( Rammus will hate you for this). At level 5 the armor debuff gives enemies standing in the area of effect -40 armor and this effect will hold on for as long as they remain in the area or 1 second after they leave it. Try placing the spell either straight on top of multiple enemies or into the path they are likely to cross for them to pick up the debuff. This will help your carry quickly take out at least one of their squishys, maybe two if they remain standing in the debuff.

You should also keep in mind that casting Spirit Fire into a bush will reveal enemies standing in it. Do this if you think it will help you get an advantage. The benefits of Spirit Fire in a teamfight are nice but not that important that you desperately have to spare the spell for the upcoming fight.

Fury of the Sands
Your ulti is definitely the icing on the cake. Besides making you look cool it gives you awesome bonuses. Upon casting you will receive a HP buff for the duration of the spell, drain a percentage of enemy health per second healing yourself for the amount and you will receive a percentage of that damage done as a bonus to your attack damage.
In short, this is something to thank the gods for. The HP buff comes in handy as a second Heal for you and can save your life when ganked or give you advantage over that overconfident enemy that made the mistake of messing with you. The area of effect damage returned as healing to you is simply amazing when it comes to tanking the enemy team. They can stand around you, hitting you for almost no damage because of your resists and you will still heal yourself from their HP. All this while not doing anything but standing there of course, now imagine dealing out damage while doing the above using a well farmed Siphoning Strike.
During the laning phase I don't use Fury of the Sands very often. It fills the role of a lifesaver during ganks very well along with your Heal and can also be used to enforce your own gank with your jungler. Apart from this it doesn't have much uses.
When engaging single or multiple enemies this is simple. Just activate it as soon as you are in the fight. It wont help you anymore when you pop it for the heal to escape a lost fight, the use of this spell is in the midst of battle when you are surrounded by enemies. Remember that the more enemies are around you the more you will heal from Fury of the Sands and can tank an entire team if done properly. With a cooldown of two minutes you shouldn't be afraid of using it. If a battle is initiated and you cast it you can take away the time you need for the battle, a recall, shopping and getting back to normal business and Fury of the Sands will almost be available again as soon as you get even close to needing it again.

Soul Eater
This passive is also a very nice thing to have. It helps you a lot with your sustain on lane and healing 20% of the damage done by your Siphoning Strike late game is great. Some people build Lifesteal items saying that this built in strenght has to be worked with. I say 20% Lifesteal is more than you should ever ask for playing a tank. The passive in itself is sufficient and Lifesteal items don't help you with what you are supposed to do.

Now to the probably most important aspect of any LoL guide, the item build. I will explain my choices especially of the starting items and the purchasing order.

Cloth Armor
This is what I usually start out with, it reduces your incoming damage from minions and champions and will significantly help your sustain.

Faerie Charm
After purchasing your armor you will have 175 gold left. Tell your team that you are going to wait at base for another 5 gold and wait for minions to spawn. As soon as they do you wait for those 5 gold, purchase Faerie Charm and follow your minions to top lane. This item is extremely important as with this and your Greater Quintessence of Vigor you will be able to stay on lane for a very long time without having to go back for shopping.

Boots
Get these next. Not much to say here.

Sheen
This is a very important item for Nasus as it significantly increases the damage you inflict with your Siphoning Strike. As soon as you get this you can start playing a bit more offensive and make your enemy wonder where all that extra damage is coming from.

Mercury's Treads
I very rarely pick other boots for Nasus than these. Magic resistance works well into your overall build and the reduced crowd control is worth gold on a tank. People will be spamming spells on you as you initiate and this will significantly reduce the duration of those slows, stuns and what not. In some cases Ninja Tabi might make a viable option though. This has to be decided looking at the enemy team composition.

If everything goes right you will have enough gold to do multiple purchases at once when you first get back to base. For me the priority is
Boots > Sheen > Mercury's Treads > Sapphire Crystal > Amplifying Tome
If you can't afford one, look for the next one in line. Looking at your available gold and the enemy jungler you can at this point decide whether you want to purchase some wards or not. Items should always have priority though.

I see a lot of other players or guides using Regrowth Pendant as starting item. I have done this for a long time as well but found that I ran out of mana sooner or later and as a result had to go to base, buy Philosopher's Stone and got a disadvantage on lane afterwards. After a good advice from a friend I changed to using the three health regeneration quintessences and buy the Faerie Charm at the beginning. This boosts your staying power a lot since you have a good regeneration rate on everything you need. Philosopher's Stone is nothing I like buying here since it only builds into Shurelya's Battlesong and I don't really like this item on Nasus. Selling Philosopher's Stone again later kind of defeats the point of the extra gold you get from it.

As soon as your first 4 item slots are filled with Cloth Armor, Faerie Charm, Mercury's Treads and Sheen you will want to buy a Negatron Cloak and a Chain Vest for overall defense. After these purchases you will notice a good increase in your tanking abilities.
Next in line will be replacing your Faerie Charm with a Warmog's Armor. The mana regeneration should not be needed anymore at this point of game. You should get some more minion kills after this to get the stacks on that armor. Another very viable option instead of going for Warmog's Armor at this point would be to get a Heart of Gold. This can later be built into Randuin's Omen and for the time being will give you a very nice 5 gold per 10 seconds along with some health points.
After this you will be finding yourself left with different choices for what to build out of your Cloth Armor, Negatron Cloak and Chain Vest. The possibilities are:

Force of Nature
I usually buy this one first. The magic resistance is nice and the movement speed bonus helps too. The health regeneration effect combined with your Warmog's Armor is just crazy. A very good item for a tank.

Sunfire Aegis
This is also a great item. The health plus armor works well paired with Force of Nature and the damage aura is a nice addition. If you have a heavy AD in the enemy team that has Madred's Bloodrazor you might want to ignore this and pick the next option. As this is also said to not be a very costefficient item it can be swapped for Thornmail or another option for this reason as well.

Thornmail
Of course this will be your choice if you are playing against one or two heavy AD carrys. Watch them kill themselves while hitting you.

Guardian Angel
This is your item of choice when you are playing against a team that deals a balanced amount of physical and magic damage.

Banshee's Veil
Buy this if you go against a caster heavy team. The health and magic resistance will help and negating one spell every 45 seconds should not be underestimated. This will always mean they have to waste one spell in order to even think about taking you down. And 45 seconds really isn't much. In longer fights you might end up catching multiple spells with this.

Frozen Heart
This can be very efficient against an AD carry. Giving you armor while reducing his attack speed can really cripple his damage. The extra mana and especially the cooldown reduction are nice too.

Randuin's Omen
Some might want to buy this. I don't since I hate having items with an active, it's a thing I simply forget. The passive and active effects make it a great choice though.

Quicksilver Sash
This is potentially one of the strongest items on a tank. It lets you throw off any crowd control and debuff spells from you. If used correctly you can block any number of spells and any type of spell at the time you choose. Banshee's Veil spellshield effect is random while this can be used to render things like Malzahar's ulti completely useless. I don't usually buy this because as said before I tend to forget items that have an active effect.


These are pretty much the options you have for building your tank. If you have nothing else to buy you can upgrade your Sheen to Trinity Force to complete the build. But make sure you have everything else bought first, the main interest in Trinity Force is the effect you already have with Sheen anyways.


One note I would like to take at this point is that I believe Atma's Impaler isn't a really good choice on Nasus. If built like mentioned above and farmed well you will deal a lot of damage and Atma's Impaler just doesn't give you much tank or a lot of damage. The damage proportional to your maximum health might be interesting but you are there to tank and not to deal damage primarily so a different item will serve you better. And believe me, even without Atma's Impaler you will slap them well enough.

During my time on lane I like to keep it casual. You don't have the power early on to aggressively approach the enemy. So try staying with last hitting with your Siphoning Strike and have the minions just outside range of your tower. This will have three effects.
1. The enemy wont be able to come close to you for hits as you can quickly slip under cover of your tower.
2. The enemy jungler wont be able to gank as you are not out in the open.
3. Your jungler can come in for easy ganks and harassment as the enemy is forced to come away from his tower.
One note on Fury of the Sands during laning is that I like to wait with first using it. Let the enemy think you are weak and that they are in control of the situation. If you don't have full health thats perfect, the enemy will try and poke you and will eventually make mistakes. When they towerdive or a jungler comes with half life you slow and bring up your ulti. They'll never know what hit them all of a sudden while they were so confident of everything.
Again, during this phase it is important that you focus on getting damage stacked on Siphoning Strike as this is one of the things that make you a powerful lategame champion.

As the game commences you can attempt doing minor attacks at the enemy tower. Push a bit and get two or three hits of Siphoning Strike on the enemy tower. Doing this constantly will eventually take it down. Of course if you manage to kill the enemy laner or if he foolishly goes back to base you can quickly try and push as much as you can. As soon as that first tower is down you can go shopping, do some ganks and return to lane frequently to get the minion kills.

During late game you will have to do your duty in team fights. This is tanking, tanking and a bit of tanking as well. I'm not saying you aren't supposed do deal out damage. A tank is a character that is enough of a threat to force enemies to attack him while being able to hold the incoming damage. Some tanks are a threat because of their crowd control capabilities. Nasus among others is a threat mainly by the fact that he can deal tons of damage if you don't take care of him. This is what you do, you go in there and start hitting. Get your Siphoning Strike on those squishys and slow down fast melee carrys. Use your Spirit Fire for some amor debuff and don't forget your ulti whenever you engage. It helps a lot with damage and healing.

I really hope this guide is of some help to people who are trying to learn how to play Nasus. Keep in mind that I do not claim to have found the one and only way. There should always be some room for changes and adaptivity.
As this is my first guide I'd be glad to receive some good comments and advice on how to improve this. I will try my very best to keep an eye on this and improve the guide as I gain more experience.
I will see you on the fields of justice!

I have added my thoughts to Philosopher's Stone and Regrowth Pendant to the items section.

As you said, this type of game is rarely the case. As this build is rather expensive you will usually find yourself deep into enemy territory when you have enough gold to finish it. In this case the game usually wont last much longer. This is a good thought for a very long game but this will rarely occur.

Concerning the thoughts of glizdka:
I have added the Quicksilver Sash (I totally forgot about this one) and a few words on the usefulness of Sunfire Aegis to the items section. I still think Warmog's Armor and Force of Nature are great choices though as the combined health regeneration is madness. One of my best games with this was attacking an inhibitor turret guarded only by Soraka when the enemy team came from behind and cought me. I managed to destroy the tower and kill four of them without assistance. Soraka managed to escape and I was able to tank the whole team. Surely that was also a good portion of luck mixed with some stupidity of the enemies but tanking this much with active regeneration and your ulti enables you to tank really really well.