Whoa, this actually sounds really kind of awesome. I've always dreamed of some kind of multiplayer game where a player can control the boss. Demon's Souls and Dark Souls had this, but I had difficulty getting into those because I admit that the games were just too difficult for me. Left 4 Dead gets it kind of close.
The idea of playing as a boss who has a goal beyond 'just kill the players and stop them from reaching point B' is what makes this the most interesting, I think. But given the very nature of the game's set up and the evolving monster, I wonder how easy to balance something like that will be. Certainly looking forward to finding out!