Secret Second Final Boss Karthus
The name is completely wacky and I wasn't able to really find any picture that could represent this well, but with how often you see Karthus skin ideas where someone has crapped on him with a unicorn (#ColorsAndRainbows), I figured why not suggest a skin idea still relevant to his champion as well as that adds a skin to the Arcade collection some more.
So far our two Boss skins are Battle Boss Blitzcrank and Final Boss Veigar, both appropriate skins. And seeing as how Karthus falls pretty well into the category of "Wow, this mage is an asshat", I think a Boss skin would be pretty nice on him. (Sure, Blitz isn't a mage, but he is AP and taking his full combo hurts about as bad as a mage. :'))
Yup though, that is kind of it. '.' I'm no artist and not awake enough to envision a skin well enough to describe it in words, so just take this very basic idea and let your imagination go wild! :D Maybe even listen to some awesome gaming music, close your eyes, and envision the artwork unfold before you! :'D Wow, wasn't that cool? :33 :D
Yeah, I'm really tired. '.'