Infernal/Demonblade Kayle

Would be a cool skin for Kayle considering the stark contrast.

She could have demonic wings, a demon-imbued sword, and demonic armor

Her Q could have her sending a black-red hell hound that would run towards the targeted enemy, having a face similar to that of the infernal hounds that Nasus spawns when he uses his ultimate.

When she uses her E she could grow horns and instead of her sword being on fire, it could have a similar dark-red color hue to Infernal Nasus spells, as well as similar animations to his E, having little black hands appearing to reach out.

Her E would also be red-black, and as for that glittering effect that momentarily makes it look as though you're glittering would be replaced with little black dots similar to Infernal Nasus' ultimate's animations.

When she ults, she could surround herself or an ally with a fiery ball with black and red flames surrounding them.