Akali - High Nuke Build
http://www.mobafire.com/league-of-legends/build/akali-high-nuke-7879

Xin Zhao - Jungle - Carry
http://www.mobafire.com/league-of-legends/build/xin-zhao-jungle-carry-18909

Renekton - Look Mommy its a - OH ****
http://www.mobafire.com/league-of-legends/build/renekton-look-mommy-its-a-oh-****-51727
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Hello!! Virtue here with my second build.

Pros
Very Strong Q hit if well farmed
Hard to Kill
AoE Armor Reduction and Damage
One of the Best Slows in the game
Very Fast at taking towers down
Looks Bad ***
Gets stronger no matter how long the game is(Can keep adding damage on to Siphoning Strike)

Cons
Requires a lot of Last Hitting Early Game
Harassments could cause you to become weak
Expensive items early on
No Escaping Abilities

Summoner Spells
Teleport: This is probably one of the best summoner spells to get on Nasus. It helps if you need to get back to your lane because you went back for an item without loosing much experience and Q farm. It also allows you to teleport to a big group of minions when they have piled up and pushed close to your tower allowing you to get a nice amount of gold and extra damage on your Siphoning Strike.

Cleanse: Another amazing summoner spell on Nasus. If you're getting ganked and they have an excessive amount of CCs this could possibly save your live. It can also cause you to get kills from someone who stuns you and is about to get out of the range of your slow(Wither). And also allows you to do more in team fights if the enemy team CCs you too much.

Fortify: I usually always take fortify. I personally think its amazing on Nasus. You may be thinking "Fortify? What does that bring into team fights?" First of all is the passive it gives you when its not on cooldown. +9 damage. Early game it'll help you get off those last hits earlier allowing more farm.(More damage on Q = Faster killing during a team fight) Also if someone tower dives you and you manage to get a wither on them and the tower is hitting them. Stick Fortify on and they just keep taking the damage allowing you to possibly get a kill. It will also make the tower indestructible if you're dead and someone is about to take your tower.

Core Items

Boots
The boots are really a matter of who you're facing and your preference. Ninja Tabi for a team of high Physical damage. Mercury Treads for a team of high Magic damage and stuns. Or Boots of Swiftness just so you can run faster.

Starting Items
First thing that I usually start off with is a Doran's Shield and a Health Potion. Why? The Doran's shield allows me to stay alive in the lane, while my runes give me mana regen and my passive heals me. And the health potion allows me to heal if someone harasses me to much.

First
By the time you go back you should have farmed enough to get the Aegis of the Legion and Boots of Speed. Grab those and head back into you're lane and keep farming Q. The Aegis of the Legion and Boots cost 2275 Gold.

Second
The second time you go back is when you have enough money to finish your boots and buy your Sheen. This should cost around 1600 depending on the type of boots you get.

Third
The Third item is you're Guardian Angel you'll have enough room for two parts of your Guardian Angel or the whole thing. So buy the Chain Vest and Null-Magic Mantle. This will cost 1100 Gold.

Fourth
Finish up your Guardian Angel and start working on Trinity Force. Get the Phage first for the extra HP boost, or if you just need that little bit extra movement speed get the Zeal first.

Fifth
You now have a choice between 2 items. Sunfire Cape, or Banshee's Viel. Sunfire Cape if they have more Physical Damage oriented champions and the Banshee's Viel if they have more magic or CC oriented champions.

Sixth
You can get a Sunfire Cape if you didn't take it as your 5th item, a Banshee's Viel if you didn't take it as your 5th item. If you don't have anyone that has taken a Randuin's Omen you can also grab that.

Early Game
Early game go to your lane and wait for the creeps to arrive. I would recommend laning with someone that can keep the enemy team away from you. Once they arrive you can start hitting the creeps until you can pop Q and take a creep kill. Don't take all the creep kills because your lane partner still needs some, but when ever your Q is ready take a kill. At level 3 if you want to push you can stick a couple of Spirit Fires out on the creep wave, but be sure to get both the ranged and the melee.

Mid Game
Mid game you should be helping you're team when they need help, but you should also still be farming your Q. Grab whatever you come across with Q unless you need to get somewhere quickly. By level 11 your ulti will have a huge effect in team fights and you should have a nice amount of extra damage on Siphoning Strike. Be sure to wither the enemy's main physical damage. Run in and hit the squishy champions with Spirit Fire and Siphoning Strike. That will have a huge impact on their hp.

Late Game
Late game your presence in team fights will have a huge effect because of your high burst damage with Siphoning Strike and your ulti's AoE damage. Keep farming your Siphoning Strike on creeps, but be sure to stay with your team. You'll be able to take out enemy Squishies with ease, while still doing a nice amount of damage to everyone else with Spirit Fire and your Ulti. If you took Randiun's Omen be sure to use it in team fights.