Boros Keyrune

The signpost cycle that every guild got: a three-mana mana rock that taps for the guild's two colors and, when you have nothing better to do with extra mana, turns sideways as a small attacker tinted with that guild's combat keyword. The Boros entry carries double strike, which is the most aggressive read of the cycle's premise, because the conversion ability is built for the deck that wants its dead-on-board mana to do something on an empty turn. The body is small and the activation is a one-shot until end of turn, so the rock-to-creature switch is less a serious threat than a way to wring incremental damage out of mana you would otherwise float into the void. The structural idea is the interesting part: a fixing artifact that refuses to be a dead draw in the late game by carrying its own minor win condition, an answer to the perennial problem that mana sources stop mattering once you are flooded. Double strike on a 1/1 means each conversion is two damage past a clear board, the kind of arithmetic that closes a game two turns sooner than a vanilla soldier would. It will never be the rock you reach for when ramp is the only job; it is the rock you reach for when you also want a body waiting in reserve.