The enigmatic creature known as Nasus came to the League from a faraway world, and served as part of a vast and learned desert empire. There he was a respected and enlightened individual, having been made the Keeper of the Great Library. However, he was also part of a race of animalistic creatures that served as the rulers and protectors of the people. Unfortunately, there were others of his kind that believed they were more than protectors. They were under the egotistical delusion that they had ascended to become gods. War on Nasus's world was never more than a breath away, as these deluded others sought to enslave the people and establish a totalitarian sovereignty. In the midst of a heated battle, just as Nasus was about to slay his treacherous brother Renekton, he was pulled through the cosmos to a summoning circle in Runeterra.

At first angered that he had been taken just as he was about to cripple the enemies of his people, Nasus' advanced intellect won out when he heard the pleas of mercy from the summoners. He learned that Runeterra was also filled with its own brand of injustice. This danger was no less threatening than what he had fought against at home, and champions were needed to protect the people of this world. Heeding the call, Nasus chose to take his place in the League of Legends, knowing that his brothers and sisters would carry on the fight back home. These days, he is the Curator of the Sands, feeling most at home in the crumbling ruins of the Shurima Desert. However, recent disturbing news has reached the Keeper's ears - rumors that perhaps his brother had not in fact been left behind. If this were the case, the destruction of Renekton would quickly become Nasus' main priority, before this world too faces the threads of his brother's forked-tongue and endless ambition.

The Curator of the Sands, though contemplative, is not to be trifled with.

Nasus in my opinion is one of the most overlooked champions available. Nasus has a good amount of natural life-steal, allowing him to focus more on items rather than spamming pots the entire early game. Damage-Tank Nasus is my favorite build by far, but by no means is the only build. This build focuses on early farming and end-game health stacking to maximize damage output.

Runes are entirely up to you. If you like to go more on the offense side rather than tanking, grab more armor pen runes. If you want a tank build then go either more health or armor runes. This is a completely personal choice, and is up to you.

I've chosen to go the tank build to help even out the damage to tank ratio. Also stacking more health will increase damage output, so it's always a good thing so grab as much health as possible.
If you'd prefer more damaging attacks, go into the offensive mastery page, rather than defense.

This is where Nasus's abilities and items come together quite nicely.
Early game I like to grab a health regen pendent to stay in lane longer, with no pots. If any, I'd grab a mana pot. After farming my Q for a while until I have around 1k gold, I'll recall back and grab a Philosopher's Stone and some boots if possible. This will ensure that I won't have to recall again until I have a nice amount of gold. Continue to farm your Q until you feel like you need to head back. By this time you will be able to finish off building your Ninja Tabi, Then, start building into a Thornmail if the enemy team is AD heavy, or a Force of Nature (FoN) if AP heavy. This is about when team fights should start to initiate. You'll have a decent amount of health as well as a farmed Q for decent damage output. When you can, grab a Randuin's Omen. This helps your allies in team fights thanks to it's nice AS debuff and your armor from the Thornmail/FoN. When you can, grab a Warmog's Armor. This is the start to your high damage output. Farm your Q some more so that Warmog's may stack up, as well as your Q. Now grab Atma's Impaler. Atma's will increase your damage output with your Q and basic attacks so much that you'll make the enemy team realize just what they're dealing with. Your Q while under the effects of Spirit Fire will be nasty, not to mention if you use your Ulti before-hand. Finish off with a Frozen Heart to ensure no one will be able to bring you down, not even an AD/AS Master Yi or Vayne. Congrats, you are now a god.

If you're initiating in a team fight, the priority list is:

Spirit Fire
Fury of the Sands
Siphoning Strike
Wither

If retreating from a team fight, the priority list is:

Fury of the Sands (This should be on cooldown if you've already been in a team fight)
Wither
Spirit Fire (While running)

I like to go with Ignite and Ghost.
I choose these because Nasus already has a slow so using Exhaust would just be a secondary W skill.
I also go with Ghost to catch up with any routing champion to Wither than Siphoning Strike them, or help me and my get-a-way.

Pros:

Loads of health and defense making him a well-suited tank.
High damage output late-game
Well-farmed Q means lane dominance.
Hits like a truck with his Q.
Just looks like a boss

Cons

If Q isn't properly farmed it will be difficult to 1v1 anyone.
You'll be focused a lot more.
Lifesteal isn't stacked, possibly meaning more recalls late game.

Nasus is a deity in the least on the battlefield. His Q (Siphoning Strike) can be one of the most deadly attacks you'll ever come to face if you ever verse a well-farmed Nasus. Because of this, playing Nasus should obviously be all about last hitting minions early game. This will cause his Q to stack 3 damage/kill. Once properly farmed, the last hits alone should be getting you aroumd 550 damage alone, not to mention the other effects that modify the damage.
Nasus without his farm will cripple him from his total possible damage output, and will hurt the entire team.