Gimli, Mournful Avenger

Counting is the mechanic here, and it does double duty. Every friendly creature death adds a +1/+1 counter, but the third resolution of that trigger in a turn is the payoff: that is when the fight trigger fires, turning an aristocrats board into repeatable removal that only comes online after you have fed the graveyard three times over. The indestructibility clause is the other half, keying off two-or-more deaths in a turn, which means the same sacrifice loops that grow the counters also make the fighter unkillable as it swings a fight at whatever you point it toward. It is a tight, self-reinforcing loop packed into a three-mana Gruul body: deaths feed size, deaths feed durability, and the third death feeds removal. The tension the design resolves is wanting bodies to die while also wanting a threat that survives to profit, and it settles that by making Gimli the beneficiary rather than the fodder. The fight is stapled to that third-resolution trigger rather than sold as an activated ability, so it fires without extra mana; you choose only whether to point it at a creature, since it targets up to one, once you have killed enough of your own team to earn it. A board built to bleed is exactly what it wants, and an opponent who lets the third one fall is the one who pays for it.