Rhovanion Rampager

Whenever this creature attacks, you may sacrifice another creature. If you do, put a number of +1/+1 counters on this creature equal to the sacrificed creature's power. When this creature dies, amass Goblins X, where X is this creature's power. (Put X +1/+1 counters on an Army you control. It's also a Goblin. If you don't control an Army, create a 0/0 black Goblin Army creature token first.)