Dreadfeast Demon

The trick is what this thing spawns. At the beginning of each of your end steps it eats a non-Demon creature and, if you paid that fodder cost, mints a full copy of itself: a fresh 6/6 flier with its own end-step trigger. Every copy is a Demon, so it can never feed the engine, only accelerate it. Two demons demand two sacrifices next turn; four demand four. The bottleneck is baked into the design, because the population of your board that qualifies as fuel shrinks with every trigger while the population that demands fuel doubles. That asymmetry is the whole tension: a runaway multiplier chained to a rapidly depleting resource. Left unattended it strip-mines your own side and stalls, which is why the card wants to be pointed at expendable bodies rather than run as a raw beater. Feed it tokens, cheap recursive fodder, creatures with death payoffs, and the mandatory sacrifice stops being a tax and becomes a second engine firing on the way to a swarm of evasive fatties. The seven-mana price and the delayed, once-per-turn cadence are the counterweights to a payoff that, given sacrifice fodder to burn, doubles a six-power evasive threat every single turn it survives.