Umbral Mantle

The free equip cost is the tell that this was built to be installed and forgotten, but the engine hides in the activation: that untap symbol turns a pump ability into a recursion loop. Pay three, fire the ability, and the equipped creature gets +2/+2, sure; the consequence is that you have untapped the creature, which means any tap-ability worth recycling can fire again and again inside a single turn. Strap it to a creature whose tap produces three or more mana and the loop sustains itself: the creature pays for its own untap and keeps growing, +2/+2 a stack at a time, for as long as the mana holds. The buff is the cover story, not the payload. This is why the card never reads as a combat trick despite what the words say: it is a combo enabler that happens to pump, a cousin to the untap-loop tradition that runs through Staff of Domination and the various "untap target permanent" effects. The flat three-mana activation is the bar each creature has to clear. A creature that taps for exactly three already goes infinite on the buff: each loop nets a free +2/+2 even when the mana only breaks even, so the size is unbounded the moment the math reaches zero. Anything above three piles surplus mana on top of the unbounded growth. The build-around question was never "does this work" but "which tap ability clears the bar."