Compare to Darksteel Myr? I'm having a hard time understanding how this wouldn't be broken. Outside of a permanent -0/-1 to the board, this will roughly shut down ground combat in two to three turns. Nevermind what happens if you build a deck around it.

Probably. You just need some non-destruction effect to negate it before it gets out of hand. Exile, tappers, Pacifism, hell even flyers make it not an issue. Now if the controller has a sac outlet, you're in trouble, but that's why it's an end step trigger

Mm. But that would still make this ability much better than indestructible. I can concede that indestructible isn't always worse. If you need to deal combat damage right now, destroying this creature might be fine. It's just a trick you can't repeat.

Also, keep in mind that exile, tappers, pacifism, etc.., are only good answers when there is one of these on the table. As soon as one dies, Master Decoy isn't cutting it anymore. Oh, and you can always attack. It's practically a 1/1 unblockable creature, since blocking will usually result in double the number attacking next turn.

I remembered Sprouting Phytohydra and was going to make some comparisons. But I figure that argument could easily be shot down by arguing that the Phytophydra is easier to combo with. Fair enough. Still seems worth mentioning in passing, though.

Phytohydra was one of the inspirations for this, actually. I forgot it was 5 mana though XD

Some cards are skill testing. Others are "Do you own doubling season?" testing :)

Anyhow; I love it. It's a much better capture of the flavour than the original hydra card is, and it takes a while to ramp up to scary levels.