Nasus is a tank/ caster that can take and dish out a ton of damage. This build is more of a tank build for when your team has a lot damage out put but doesn't have a good tank. But dont worry with Nasus' abilities you will still get kills and have your enemies running in fear.

This is my first build so comments and suggestions are much appreciated. This is how I build Nasus and it has worked pretty well for me so far, and i hope the build helps.

This is a great starting item that helps you not be so squishy early game
You will want these boots because it will give you some MR and reduces stuns and slows, but you can also get boots of swiftness if you want more movement speed.
This is a great item for Nasus because it gives you mana and gives you extra damage when you cast your spells.
I like to get this vary soon to turn nasus in to a tank right away, but you can switch the order and get thornmail first if the other team does more AD than AP, or if your laning agaist AD.
Thronmail is great against AD and almost stops them in their tracks.
I get this next because now that you have armor and MR now you need some health to make u the ultimate tank.
After you have a lot of health use this to add some more armor and AD.
If the game makes it this far this will add more MR and some AP to your spells.

This is a tank build that will help your team to victory, you want to get the Siphoning Strike first and last hit minions and it adds 2 damage to the spell every time you get a last hit with it. Late game your going to want this damage high so you can hit like a mac truck.

Then get Spirit fire to farm minions and keep the other champions from getting xp.

Your wither is one of your best spells because it slows enemy champions a ton, you can use it to case, to get away, or to stop AD dps and win a team battle.

Your ulti is one of the best tanking ultis in the game, it increases your health and starts a beast aoe around you that shreds enemies health. but is has a long cool down so use it wisely, like in big team fights.