Carapace

The second clause carries the design: you pre-pay a regeneration shield onto a creature so the protection sits on the board, ready at instant speed the moment removal or a losing combat exchange threatens the host. No mana held up turn after turn, no reactive instant clogging your hand. That sacrifice-to-regenerate template answers a real awkwardness in green's toolkit, which has always favored durable bodies over reactive answers. The cost is the cost every Aura pays: a two-for-one if the host dies to anything regeneration cannot stop (exile, bounce, a sacrifice edict), since the Aura follows its creature to the graveyard. The +0/+2 is the consolation for that risk, padding the body against small burn and tilting a few combat math problems in your favor while the regeneration waits in reserve. It is a thoroughly modest design, the kind of cautious green common that defined its era's approach to creature protection, back before regeneration fell out of favor and indestructibility, hexproof, and the various damage-prevention shields took over the same conceptual space.