OP.
Right off the bat.
It breaks one of the fundamental rules in the game, only 6 of each creature in a deck. 12 with PUs. You can just sit there an spawn things all game. I mean, imagine a field full of devourers?
Suddenly you don't have to upgrade every card to have a fully upgraded dark deck. Just 6 devourers, six of these, some steals, some drain lifes, some nightfalls, and pillars. Dropping six creatures a turn?! And upgraded! WAYYYY OP.
Not to mention that on the off chance you get a creature you don't like, you have an infinite supply of parasites to kill them off. Sorry kid, that's just crazy.