Skin Idea: Demonblade Kassadin
This is not my video Riot, so don't remove this post. It is simply a video from a youtuber and I did not in any way tell anyone to subscribe to him in this post.
Demonblade Kassadin is based off the newest Doom game, more specifically, the Cyberdemon. You may ask why a skin based off of said demon but while I was watching a video by Markiplier in which he fought it, the demon pulled out a Kassadin-like sword and slashed Mark with a very close looking Force Pulse. So the combination of him having Kassadins weapon (not all the time but it's there), they both have horns, and he used Force Pulse, I think this would be an awesome skin for Kass for 975-1350 RP. 
His Q (Null Sphere) would be reddish-orange and give him a nice looking shield of hellish looking origin. His W (Nether Blade) would be bright red when activated, while his regular blade would be a dark reddish-orange color. His E (Force Pulse) Would be reddish-orange like I already said. And his ult (Riftwalk) would be just a red blur really. He'd be red with dark armor and horns and his passively generated ripples on the ground would be red and maybe can sometimes leave a hellish looking symbol? It could still just be random lines but symbols would make it look cooler.
Anyways that's my idea and I think it'd be a horrifyingly great skin to have. Imagine a demon coming at you from behind a wall and sending you to hell!
(P.S. Go to 13:30 and watch for a few seconds, you'll see him do force pulses and you'll see his firey blade)