Midnight Covenant

The repeatable : +1/+1 ability is the Shade mechanic, the same mana-sink that powers Drudge Skeletons' cousins and the long line of Shades stretching back to the earliest sets: turn untapped Swamps into a creature that grows by a full point each activation. What this Aura does is detach that ability from the body it usually lives on and let it ride any creature you choose. That portability is the appeal (bolt it onto whatever attacker is already getting through and you have a Shade for two mana) and the liability (kill the host and the opponent collects a clean two-for-one, since the Aura dies with it). The pump cost is open-ended, so the ceiling is whatever black mana you can leave up and the floor is a single pip for one point: feast when you are flooded, dead weight when you are screwed. What it conspicuously lacks is the bonus that usually pays the Aura tax, an enters effect or evasion granted on attach, which means the host has to matter before the pump does. The result is a narrow accelerant rather than a threat in its own right: a way to convert surplus black mana into combat math on a creature that can already connect, with all the fragility that comes from parking a growth engine on a target your opponent can answer in a single removal spell.