Mark of Sakiko

Combat damage as a mana spigot is the whole gambit here, and the unusual clause is the one telling you not to lose that mana as the turn's steps and phases end. Most mana floats and then evaporates between the combat damage step and the second main phase; this Aura punches a hole in that rule, letting a connecting attacker dump a payload of green directly into your post-combat play. The bigger the hit, the bigger the pool, which means the card scales with trample and pump rather than with a fixed value: a creature that gets through for seven leaves seven green waiting once combat resolves, ready for a fattie, an overrun, or an X-spell that closes the game the same turn. The cost of admission is the cost of any combat-trigger Aura: it does nothing until the creature lands a blow, and a chump block or a removal spell on the enchanted attacker spends your card for nothing. It rewards the deck that was already going to connect and just wants somewhere to put the mana that connection generates. As a piece of mana theory it inverts the usual ramp question: instead of paying mana to make a threat, you let the threat pay you, and the spent-mana exemption is what turns that payment into something you can actually spend before it drains away.