If it was 1 turn for every creature i'd say it's balanced and promote it.
I think you don't realize how strong this can be.
Let's take SPlat for example: part of why it works is that you can, in the same turn, cast the scorpion, give it momentum (screwing up any shield compeltely) and apply the neurotoxin, dooming your opponent forever unless he has Purify. With a Labyrinth, you get just the extra turn you need to RT/kill the otherwise-deadly scorpion with whatever you please.
Same goes for rush decks, which, just because of you having this in play, aren't nearly rushing anymore.
Same for fractal decks, and pretty much every other deck using creatures.
The concept is thematically awesome, mechanically strong & sort of innovative, this is thus a good card. I just think the balance isn't nearly okay yet.
I suggest a cost increase and/or making it only last 1 turn whatever the creature is.