Have the creature die at the end of the turn.
That would help balance it more, and make the dark/death synergy (I love this word, thanks for it Scyther!) even more brutal.
awesome idea! +1 karma glitch (edit: since i gave +1 karma already in past hour to someone, i can't quite give it to you now.. but i will later!)! This totally works with the whole idea of them being called out and crawling out (and then crawling back into cavern/depths)
Here is how I think it can work.. 7 as summon cost, then pay 2 dark quanta to have a random death or darkness creature come on the field however it will have only 2hp and have 1 poison counter (aka, it will normally die after turn 2). This will be enough for it to use it's active ability once but you won't aggregate a huge army of devourers/etc :p
Thanks glitch/scyther!