Which ability, going by the flavor text, do you think is the scariest/craziest? I've always considered 
"Wither" to be pretty frightening. He literally ages you until you become a frail, elderly person. And we all know old people have low movement and attack speed.
Devour
Organic Deconstruction/Life Form Disintegration Ray
Frozen Tomb? Normally you could not free yourself from it (gameplay purpose), because true-ice or smth.
Edit: Also 
Noxian Guillotine.
Starfall: "Soraka calls down a star at the target area"
Ok, please continue destroying our world.
Test of Spirit: "Illaoi rips the spirit from a foe's body, forcing it to stand before her. Spirits echo a percentage of the damage they take to the original target."
So when will that Soulstealer Illaoi skin be coming?
Transfusion, just the thought of my blood being sucked is a bit eew.
old people have low movement and attack speed.
How do you know they have low attack speed? :^)
Massacre?
Void Rift : Can casually OPEN A PORTAL TO THE VOID which you could probably fall into if it wasn't for game mechanics.
I find 
"Rend" kind of scary. Spears being ripped out of me? Ouch!
Aqua Prison
Because who isn't afraid of being trapped inside a globe of water until you drown? (Drowning is extremely painful in case you didn't know)
R, obviously.
Crowstorm.
la dee da just taking a nice stroll though midlane...
OH HELLO THERE
[sg-ahri-1]
