Boomstacker is a rare Creature — Goblin Artificer card from Unsanctioned (UND), collector number 49. Current market price: $0.24, TCGplayer market prices via Scryfall.
As this creature enters and whenever it attacks, stack two dice on top of it. (All dice must be stacked vertically, one on top of another.) This creature gets +1/+1 for each die in its stack. This creature attacks each combat if able. When the stack falls, sacrifice this creature.