This being my first build I am not entirely sure how people will take it but figured I would throw some ideas out there and hopefully it helps people out. That being said Nasus is a great champion I have had a lot of success with using this rough build and I hope it can help you as well.
with the right runes is naturally tanky
can deal some heavy unexpected damage (lost count of how many times a warwick or ad champ thought they could take me on early only to get wrecked and give me an easy kill)
great in a team fight
good sustainability in lane ( a passive that gives you 10%/15%/20% increasing at levels 6 and 11 yes please)
easy to learn
going with above no great harass
dependent being able to farm ( if your kept out of range by good harass this can be trouble nothing you can't come back from though)
The reason I chose the runes I did is because magic resist is amazing to have early. It tends to negate the effects of alot of champions abilities in the laning phase and gives you some additional tankiness mid/late game which in many games after getting treads and Guardian Angel I have not needed any Magic Resist items to protect me from the enemy AP carries freeing up a very valuable item slot. The masteries are just typical tanking masteries not much to really say about them.
For skills I start with one in siphoning so I can start getting those minion kills in it and start ticking up the damage counter in it. Now I personally only need one point in it to start farming it up but if your new to Nasus its hard to judge when you need to go for a last hit. If you need to put an extra point or two in it that is fine just adjust as needed. A lot of other Nasus players don't prioritize spirit of fire so I will explain while i do. At level four spirit can usually take out the caster minions and make it extremely easy to pop a melee minion with your siphoning strike and last hit the remainder. Spirit is an amazing pushing and farming tool especially if you are solo top which is a role Nasus excels in. Using spirit over a wave of minions gets you some good cs at least three a wave from the casters once you get it up and usually keeps your opponent from being able to last it your minions. If your opponent is foolish enough to stand in spirit go ahead and pop him with a siphoning strike in it and he will learn his lesson. Now there are some arguments for prioritizing your siphoning strike I recently went solo top vs. another Nasus doing this and though he had his siphoning farmed slightly more than i did by the end of the laning phase i was able to easily push his tower down and come out with more cs and putting my team at a tower advantage and in the end that is what the game is about.
Early game/laning phase
When I usually decide to pull out Nasus is when my team needs someone tanky to hold down a solo top role so that is what I am going to talk about here. No matter what however there is only one real goal you should have for early game 1) farm your siphoning strike get its damage up and bring some unexpected pain. Now with this are the always important goals of the laning phase which are do not feed and take out the enemy tower. This being said onto the explanation. When you first see your opponents team composition start thinking of the most likely opposition your going to have top. Is it going to be a 1v2 or 1v1? what type of damage are they gonna do? what abilities are they gonna scale first and how is this gonna affect my ability to move around. If you are 1v2 I recommend getting a regrowth pendant and health pot to start off. If 1v1 depending on who you are going against either get a mana crystal, if it is either a magic damage dealer or someone who isn't a strong solo champion, but if you have someone with AD and some good harass throw down a regrowth pendant again. In either case this is the way I usually carry myself in the lane. If you have a 1v1 and are confident don't be afraid to be agressive your spirit of fire combined with a hit from your siphoning strike can deal some good damage but use this combo sparingly, spirit fire isn't cheap on the mana cost wait till you level it up a little to get maximum effect out of it, and ensure your hitting enemy minions with the spirits AoE as well I usually hold it off till around level 4-5 on this when your getting a little bit better damage out of spirit. You should be able to dominate the lane once you have 3-4 points in spirit fire as this wipes out caster minions what does this mean? It means your gonna force your opponent to make one of two choices 1) let minions and the occasional siphoning strike, that is right its one of the few abilities that hurts enemy buildings, or 2) start taking minion hits which will set you and your jungler up for an easy gank. Now for the 2v1 the name of the game is play it safe. In this situation farming your q takes a backseat to staying alive and keeping your turret healthy till you can get a gank and here is where maxing spirit fire once again is so much better than get siphoning up first. If you are having problems with mana because your overusing spirit fire a good recipe is use it on waves with cannon minions or when your tower is in jeopardy of having some hurt put on it. All in all 2v1's are no fun especially if the other team has some good harass the best you can do is try to stay in exp. range without putting yourself at risk an easy way to do this is dance pay attention to when an enemy minion is about to die and roll forward into range as it does and out quickly after repeat this but once again PLAY IT SAFE you lose out on more exp when you die, inconvenience your jungler as he has to come defend or risk losing the tower, and give the enemy a nice chunk of gold.
Now I don't think I should have to explain it but I am gonna go ahead and show you how to lay out a succesful gank between you and your jungler. Communication is key let him know if that bush is warded so he doesn't waste his time sitting in the bush waiting for an overextension that will never come or counter gank that will cost him his life. Don't rush it tell him to wait a second for them to push forward a bit then throw your wither on them and eat them. Plan ahead if you know your jungler is coming up for a gank in the next minute or so let them push gradually forward so they think they are gaining an advantage as waiting till the last minute to let them completely roll you on a wave will usually give away that something is up.
If you have followed my recipe and played it safe you either have 1) In a 1v1 successfully pushed down your opponents tower and asserted your authority as a boss or 2) in a 2v1 kept your tower alive, farmed decently, and with a competent gank were able to gain a nice advantage there turret may still be up but the next phase of the game has begun. If this isn't the case never fear just keep playing to the best of your ability and alter your build and playstyle accordingly. At this point in the game I usually have my trinity force, merc treads, and my philosophers stone if I went that way and am well on my way to Guardian Angel ( on the note of guardian angel start on what you feel is more of a threat on the enemy team if its ad pick up the armor items while ap null magic mantle). At this point you should be getting to the first team fight or so and here are where things get fun and reason number two why spirit fire is so amazing. You are either going to initiate or if someone else does charge right into the middle DO NOT trigger your ult until the enemy team commits itself to the fight. Your primary concern and what I like to do is throw wither down onto the enemy Dps/AD champion at the beginning of the teamfight as it lowers his effectiveness and wither should cooldown in time to slow another enemy and either mop up survivors or save a fleeing team member. Your also gonna want to throw down spirit fire and here you want to do two things 1) maximize who its going to effect and 2) make sure its over whoever your ad carry is focusing. Why you ask? For those who don't Know already spirit fire reduces an enemies armor by 20/25/30/35/40 depending on what level in addition to dealing great damage both in its burst and per second which means the champion getting targeted is going to melt. Continue with this method through each teamfights throwing in your q when possible and your team should be in good shape if they are competent giving your team better map control, some turrets, and a dragon or two.
By this point in the game the enemy team fears you and rightly so. You have dealt some unexpected damage pulverizing squishies and demolishing cocky carries who think they can 1v1 you. You are an engine of tanky dps destruction your siphoning strike devestates turrets and inhibitors alike. They have two choices focus you and watch as your carries melt them or dont and watch as you smack there squishies upside the head with siphoning strike. That or your desperately defending your base from an encroaching enemy because your team had a run of rough luck and lost a team fight or two either way all hope is not lost for either team so neither give up hope or get cocky. There is not much change in the battle plan throw your AoE down were its gonna have most effect pound people into the ground with your siphoning strike, pop your ult when the enemy is committed and slap a wither down on fleeing champions who think they are safe but are just in range of one of the best slows in the game.
Items basic explanation
Now alot of people like phage early on nasus and I can see why. A good amount of health, some ad, and a slowing passive are all nice. I personally go for sheen because the extra mana makes it alot easier to throw your spirit of fire out more often which is key and your siphoning strike plus the bonus from sheen makes farming your q pretty easy business. Another item I like to start with solo top is regrowth pendant which you would then turn into philosophers stone. This gives you good regen for both your mana and health and is extremely helpful if you know your gonna have a pretty annoying lane or are just learning and want to play it safe. I go for a Trinity Force first because if you follow the runes you usually are pretty tanky without any tank items early, it is an all around great item and the speed boost really helps to mediate one of Nasus' main problems which is the ease with which he is kited. After that I enjoy picking up a guardian because it gives you all around boosts to defensive stats and that second life is a game changer in alot of cases. With Guardian and Merc treads you will usually have about 152-157 in both armor and magic resistance which negates around 67% of both types of damage you will be recieving. From here its a larger personal choice on what you build. Heavy AP team grab yourself a Force of Nature or Banshee's veil. Heavy AD Randuins, Thornmail, and Frozen Heart are all good options.
Items a little advanced
Now I am going to explain why I put the additional non-core items. Personally for magic resist I love Force of Nature it gives great health regen, the speed boost helps to once again address Nasus' kiting problem, and a massive chunk of magic resist. Reasons you might want Banshee's over this is 1) the extra health and mana it provides are good 2)cancelling the effects of one spell every 45 seconds yes please. If your strapped for cash but need some good magic resist in your item grab yourself a quicksilver sash its active, removes ALL debuffs from your character with a 90 second cooldown, plus the magic resist are quite nice for the reasonable price of 1440 gold. as for armor I love Randuin's omen. This item fives you a nice 350 boost to health, a good chunk of armor, health regen, a chance to slow passive, and an active that slows everyone within a certain radius when activated which adds some nice cc. Frozen Heart is another great option which gives you a large armor boost good cooldown reduction and slows everyone around you not to mention plus 500 mana. I just think Randuin's, despite a higher cost, is better and here is why 1) you need the health more than you need the mana in my experience 2) the increased cost is not a huge deal once you get Nasus down.
advanced items cont.
Once again if you are strapped for cash a good armor substitute is Thornmail. It gives you the highest armor of any item in the game and its passive, On being hit by standard attacks, returns 30% of damage taken as magic damage, is the bane of any pesky dps champion. Now onto my last item choice Bloodthirster. This item is overkill pure and simple if you have farmed your siphoning strike properly you can usually heal for about 1/5 of your health in one hit. It is by no means necessary but forces the enemy to either focus you in the late game teamfights or have their squishies forced to either back off or get smacked around by your devestating siphoning strike and autoattacks. All these item are just what I usually end up with when facing a balanced enemy team it is in no way a rigid system of, "build this or you shall fail as Nasus." Build for the situation if your enemy has no AD dont build a Randuins or Frozen Heart. Instead be happy that they were foolish give you more build options.
this is my first build as I said so criticism is welcome I will be updating/editing it alot more. I really just wanted to get it out there for people to see so they can get some ideas for themselves and hopefully teach people how to play an awesome champion.