Call me crazy, but i have this theory after reading "Necronomicon". In the short story "The call of Cthulhu" (Cthulhu = C'thun), Cthulhu is described and known as appearing as human, dragon, and some octopuss-alike creature. So obviously my guess, is that Cho'gall were able to ressurect some spark in C'thun, that somehow corrupted Deathwing, making him a part of himself, a vessel, -------- Deathwing & C'thun is the same, but this will be revealed first in the Deathwing encounter.
So last boss = Deathwing -> C'thun in a phase change. Wild guess...
Edit: -> This is when taking into account that Deathwing has in some points of the game tentacles, and that himself and twilight cultists have mentioned higher beings as their leaders.