Make "Little Devil Teemo" cost 666 (bundle it with smth else to match tier cost)
It is another thing many players want to see. Making "Little Devil Teemo" cost 666RP is not applicable due to amount of work put into skin but you can workaround this by selling smth else with it. Basically skin will still cost 666RP but you cannot buy the skin only. Sinister. :D
For example with 6 Hextech chest (wout keys because it is sinister). Maybe stylized chests (with sharp edges and in different color scheme) and keys (trident). Or with 6 chests or 6 keys at random to make it even more sinister.
UPDATE Making skin cost 666RP straight will render it less valuable and less unique.
UPDATE2 Judging by the number of downvotes and lack of arguments people just want a cheap high quality skin... Eeeh... I'm actually ok with 1350RP cost. :)