Nasus: Uber Support Tank
This is my first build here on Mobafire so if you see anything that you think would be helpful let me know!

-This build is intended for teams that already have a primary tank. The build focuses on high survivability end game and amazing team support during lane phase.

Lets get started!

EARLY GAME: This build focuses strongly on assisting teammates during lane phases. Keeping them out of harms way to get last hits while also building stacks on your siphon strike.

MID GAME: As mid game progresses Nasus should be a part of most ganking and defensive stands. His wither exhaust combo can single out a weak enemy for an easy kill.

LATE GAME: Late game is where Nasus shines. Becoming nearly unkillable he can solo defend a tower verse a 5 man team for a short period of time, and his siphon strike can turn a close 1v1 into a big LOL.

PROS AND CONS

Pros - - -> Late game almost unkillable
Awesome lane partner
Can easily farm solo mid if need be
Great farmer early,mid and late game
Great CC when used with proper summoner spells

Cons - - -> If he gets caught alone he can't dps himself out
Mana issues if a team fight goes on too long
Countered by champs like Jax and Xin Zhao be careful
(build specific): could have issues with high AP chars in end game

I prefer Exhaust and Flash.
-Remember one of the keys to being a support tank is SUPPORTING lol. Having a perfectly timed exhaust can make or break a gank and being able to flash into or out of a team fight is a huge plus as dying as a tank is NOT a good thing :(

Other good spells you could use are Teleport or even Fortify to be able to get to a teammate fast or hold a turret if your alone or no one is near it.

DO NOT USE - - -> Clarity - even tho he has some mana issues, not taking a support skill over a personal skill is a no no in this build.
ALSO DON'T USE - - -> Rally/Clairvoyance/Ignite - leave the DPS buffs and damage output to the carries and unless you're playing high end ranked games CV is useless.

Nasus' skill sequence isn't as important as a lot of other champions. The one thing that you NEED NEED NEED to do is make sure you get your siphon strike FIRST! it doesn't matter if you're solo mid, or in a double lane. You need to get last hits with your Q ability as fast as you can early on in the game.

I recommend taking the siphon strike upgrade whenever its available and leveling wither and spirit fire alternately as the game progresses taking your ultimate when it is available as well.

Okay here is the bread and butter of our Nasus build.
-When you start the game you want to buy a Sapphire Crystal and two Health pots
-On your next trip back get your boots: Ninja Tabi and if you can afford to upgrade your crystal to a sheen do it, as it will be important when filling your last item spot.
-Now you have your boots and Sheen and can start working on the heavier items in the build. You're going to want to rush to Warmog's Armor to start building your health ASAP. (The order in which you build Warmog's is up to you :)
-Once you've got your first 3 slots filled with Boots, Sheen and Warmog's it's time for Frozen Mallet. Your health should be pretty legit by now and you should be present in every team fight as a nice buffer for your carries. (again the build order for the mallet is your choice)
-Next is the Sunfire Cape. Not only does it add even MORE health but during those mid to late game team fights it can put out a good amount of damage for assisting your team "support tank" :)
-Now that you've got your HP has some awesome item bonuses its time to take advantage of the sheer amount of health you have. Buy an Atma's Imapaler since a percentage of your health is converted to attack damage. (I love this item on any health stacking champion!)
-You should now have 1.boots 2.sheen 3.Warmog's 4. Mallet 5.Cape 6.Impaler. However youre not done yet. Upgrade your sheen to Trinity Force which will complete your support tank item build!

NOTE: If your tank is having a hard time or just too squishy you may want to just sell the sheen at the end before upgrading and pick up something like Thornmail or Guardian Angel.

I use a basic 9/21/0 build which I've found is interchangeable with a lot of support and off tanks. If you use the same summoner spells i show in this build you cant go wrong with the mastery tree.

For quintessence i use 3 Quintessence of Vitality It allows survivability in early lane phase as well as adding to your health end game to stack with Atma's Imapler.

For makes i choose 9 Marks of Warding These play in nicely since the build doesn't focus around magic resist. Remember you aren't the main tank on the team!

As far as seals go i use 9 Seal of Vitality further establishing the early and late game HP advantage.

Last but not least are glyphs. I use 9 Glyph of Focus This allows Nasus to use his siphoning strike much more often and combined with the mana in the first item purchase you can spam siphoning strike and get stacks going fast!