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Brutalizer + Haunting Guise = Ghastly Mask
Cost : 1337 + 1485 + 503 (combine cost) = 3325
(Honestly, this part Riot would have a better handle on...I'm sure they have a formula for how much stats cost, and its not worth bothering to figure out exactly what it should cost also I added 3 to the combine cost, because ending in not 0 or 5 is annoying)
Picture: Some sort of brutal Ghostly/ghastly mask. Maybe purple like ghostblade.
Gap it fills: Defense Penetration for hybrid champions like Jax, Kayle, etc.
Alternatively it could do 20-25% ArP/MPen instead of flat. Kind of makes sense, since early game you could use the flats, but late game the %'s would be better, when you can afford an item that costs 3300g.
Possibly having flat MR pen and % armor pen would be best, considering champs naturally accrue armor as they level.