Yahenni, Undying Partisan

Two abilities pulling in opposite directions, sharing one body. The growth clause wants opposing creatures to die: it rewards you for trading, for board sweepers, for grinding combat into the dirt while Yahenni gets bigger off the wreckage. The sacrifice clause wants your own creatures to die, and it answers the obvious question that arises the moment a creature starts accumulating counters: what stops the opponent from simply killing it in response. The indestructible activation has no mana cost, only a creature, so as long as you have a body to feed it Yahenni walks through targeted removal, board wipes, and unfavorable blocks alike. The two halves form a closed loop: the more the game devolves into attrition, the larger Yahenni grows and the more fodder you accumulate to protect it. Haste closes the gap, letting a freshly cast Yahenni start applying pressure before the opponent has set up the trades that feed it. The Aetherborn typing is incidental; the Vampire half is the more telling tag, slotting this into a long line of black creatures that treat death as currency. What distinguishes Yahenni from the sacrifice-payoffs around it is that the engine runs on either player's losses, and the protection is self-funding rather than rationed by mana.