Here is my Build for my favourite champion "Nasus". I love him because he has unlimited power, has a slow, AoE and a crazy Ult that scares the **** out of everyone.

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.

Level up in this order "R" "Q" "W" "E"

R: Ult above all else
Q: to gain power
W: his slow is great for focusing Carrys and also in the lane phase you can keep people away from you if they attack you in lane, great escape ability
E: Great AoE but very mana dependant so only use if need be

I chose these runes because...

Nasus is an Off-Tank, so build him tanky with his masteries (and runes) and then build ultity also. This is because you can build him DPS/Off-Tank if it is needed.

TELEPORT!!!

Nasus needs to stay in lane to power up his Q

Start:
Get the Regrowth Pendant so that you can lane
Early Laning:
Mid Laning:
Late Laning/Ganking/Team Fights:
End Game Team Fights:

DRAGON
BUFFS

Power up Q so that even though you are tanky, you can still do lots of damage