Everyone always talks about how Darius or Garen will "1 shot" any squishies but that's not really true, Garen will 1 combo them if they're the villian and Darius if he gets 5 stacks but they still have to stick around to get combod.
Nasus redefines the word 1-shot. It's not like getting killed by Leblanc or Zed, when you're just peacefully farming bot lane and Nasus comes out and presses Q on you, you don't just die. The sky tears asunder, the heavens shake, and mountains fall. You no longer hold any worth as a champion. Any items you have built might as well have been sold. Your turrets are forfeit, and the amount of time left in your game rests on his whims. When you drop a cup in the kitchen and hear the "thunk" you begin to sweat and see dropping health bars all around you. You lie in your bed telling yourself that everything will be alright, that maybe if you 1v5 him you'll be able to kill him with minimal casualties, that maybe you can bring this game back. But it's no use. You are nothing more than a walking number 6 now.
Also wither is a pretty good skill too I guess.
