First of all Nasus is one of the most overpowered champions in the game, according to his ulti.
His Siphoning Strike is a seriously bad *** ability. It have tons of damage, if you use it right. Nasus is not the best laner, and his aoe is not for damaging, but it can help you push a lane a bit.

Note: ALWAYS LASTHIT MINIONS WITH SIPHONING STRIKE (get as many minions as you can it is very important for all champions, but especially for Nasus(Veigar and Sion).

Nasus is a bruiser, but i play him as tank, because his Siphoning Strike has a major damage output if he last hit minions. Damage items aint useful then.

The only problem Nasus has is mana regen, it is not that high. You can buy Charlice Harmony, but i dont do that, cause my build is kinda expensive. Instead i recall a little more.

Spellchoose: Q(1)-W(2)-E(3)-Q(4)-Q(5)-R(6)-Q(7)-W(8)-W(9)-Q(10)-R(11)-W(12)-E(13)-W(14)-E(15)-R(16)-E(17)-E(18)

Siphoning Strike (Q) : Main spell, it is very important for Nasus damage output focus ranking it for high damage.

Wither (W) : A damn good spell for engaging combat, slow the squisy champions and set Spirit Fire under then, then you are pretty sure to harrase them. Just remeber that the slow aint alot at start it builds up. And it also slows attackspeed.

Spirit Fire (E) : A less usefull spell, it haven't that high damage, but it decrease armor, it is good against tanks (example rammus).

Fury of the sands (R) : One of the most overpowered spell in the game (if you are 1v1) it heal you when you use it, it does aoe damage around you in percent, you get more and more damage and enemies fear you! ;D
But dont use it to harrase, thats a very bad idea.

A good idea is to enter a fight by using the combo:

->Spirit Fire ( place it smart , they way they are passing or so the champion are in the middle, ALWAYS place it so they are in the aoe)

-> Wither (when they are in the middle of the circle)

-> Siphoning Strike (when you are on the way toward the target champion, use it as soon as you have casted wither, then you have the oputinity to use it several time. That means massive damage, if you have farmed alot of minions)

-> Fury of the sand (you ulti is not always needed, only use it if you get ganked by 2 or the enemy champion are feeded or have other major benefits. Just be sure not to waste it. Just a hint: When youre fighting someone without ulti on, if you are about to die use it fast! Then you can overwhelm the champion and get him with wither.

The think you have to know is that i play him tanky. And when i said tanky i ment the item build.

The masteries are: 24/3/3

Offense are: Deadliness (3/3), Cripple, Archmage's Savvy (3/3), Sorcery (4/4), Alicrity (2/4), Archaic Knowledge, Sunder (3/3), Brute Force (3/3), Lethality (3/3), Havoc.

Defense are : Hardiness (3/3)

Offense are : Perseverense (3/3)

You can build him more tanky it is up to you.

Im not "runeing" for Nasus, my normal build is pure damage, which i regret. You can take runes builds from other guides, im not sure about the exactly runes. Just do not buy caster runes!

Here is a guess, but as said im not into rune building.

The items i build is:

Dorans Blade - Ionian Boots of Lucidity (for faster minion farm with Siphoning Strike) - Guardian Angel (for survivabilty, and it is good to have for reviving spell) - Rylai crystal scepter (for slow, but most for ap and health. You slow with wither so it is not necessary with the slow from Rylai) - Sunfire cape (for health and armor, it is good for minion farming. And IT STACKS GREAT WITH THE ULTI) - Banshee's Veil (MANA! Is needed for Nasus, but health and spellblock is also okay) - Hrabadons deathcap/Aegis of the legion (Normally i have won or lost here, but if you get to it, these are the 2 that i would choose between. You decide: Tanky item or damaging item? Replace it with dorans blade).

I normally use Exhaust and Flash for my summmoner spells.

Flash : A must, flash is "The summonerspell", good to enter a combat and flee.
Exhaust is not that usefull, because you got wither, but using it on the second guy if 2 are fleeing from you, cause of your ulti. Wither+Exahaust make people litte ponies, they cant do damage and cant run (If you get the right mastery).

There is two things in teamwork:

Always use Wither on the most squisy champion and they are nealy dead when you come and gank them with you friends (an ashe or caitlyn for example), you can stack it with spiritfire aswell = more damage and less armor.

Second thing is: Always use Spirit Fire on the tank if he is stacking armor!

Thanks for reading my guide and hope it will help, comment and rate :)

Ps. Please say if there is any bugs/errors in the guide i should fix.