Horn of the Mark

The trigger condition is the whole engine here: two or more attackers, and you dig five deep for another body to add to the hand. That gating rewards a specific kind of deck, the go-wide token or aggro shell that already floods the board and swings in packs, and it does little for the grindy midrange plan that often wins by attacking with a single large threat. Card advantage tied to combat is an old idea, but most such effects care about damage dealt or bodies dying; this one cares about the width of the attack itself, refilling your hand precisely when the board is already ahead and the hand is threatening to empty. The dig is a soft draw rather than a guaranteed one: five cards seen, one creature taken if you find one, the rest bottomed randomly, so a deck light on creatures whiffs the trigger as often as it converts. That is the balancing math. It never finds a spell, never finds a land, only ever a creature, which keeps a two-mana artifact pointed at exactly one thing: the next attacker for a board that keeps coming, not a general-purpose card engine that would let the horn refuel anything. Legendary status caps you at one copy in play, a quiet nod to the horn as an object rather than a keyword, and every qualifying attack fires it, so the payoff compounds turn over turn as long as the swarm keeps arriving.