*Long Post Incoming*
I'll never understand why more people don't play AP Nasus. It makes no sense whatsoever. Let's look at the facts:
-Top lanes consist of champions with Sustain, Escapability, CC, Mixed Damage, innate defenses or defensive builds. This is where bruiser Nasus fails. His Q damage can be sustained early on so he loses trades.
-Nasus' best lanes are ones where he can itemize against it and farm his Q safely.
-Nasus wins lanes he can out sustain and out trade.
-Nasus Q will grow in strength regardless of him going AD/AP. Bruiser Nasus builds all tank items so he won't have any bonus AD anyways.
With these facts in mind let's look at AP Nasus.
-AP Mids are very easy to itemize against. Becuase of his ability to stack MR he removes burst champions from the game.
-AP Mids lack sustain. They can't take a Nasus Q and recover without potions. So he out sustains and trades them.
AP Mids don't have armor. Your Q, even without Armor Pen or AD does immense damage.
-AP Nasus can farm Q easier than Bruiser Nasus but also has a second form of damage in E and his ultimate gets stronger.
-Last and most importantly is this: AP Nasus is the single greatest farmer in the game. Better than Morgana. Better than Karthus. His Pool is larger than Morgana's and hits all 6 minions every single time. You can farm mid, your wraiths, enemy wraiths, and never miss out on a single CS. Since you're not dependent on Q you can spam it and since mid lane is so short and you're so tanky, ganking AP Nasus is nigh impossible.
AP Nasus brings all the benefits of Bruiser Nasus without the litany of flaws. He only has 2 counters mid and those are the 2 AP mids with the mix of CC, tankiness, and sustain: Ryze and Swain.
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Realo won gold at the Sex Olympics with a BROKEN FRIGGIN NECK.-Voltch