Cloister Gargoyle

Blocking and descending are the same activity here, and that overlap is what gates the payoff. The gargoyle enters as a 0/4 wall and takes its first venture step on arrival: one room down, several to go before the +3/+0 and flying come online. The defensive job and the payoff condition draw on the same turns; you want to be stalling on the ground while grinding through rooms, so every turn spent as an immovable body is a turn spent inching toward the descent's end. Completing a dungeon (any dungeon, once) locks the buff in permanently: the ability checks whether you have crossed that finish line at some point, not whether you currently occupy a completed state, so no later dungeon can revoke it. On its own, though, the descent crawls. A lone entry trigger advances a single room, so the gargoyle leans on outside venture sources to close the tally faster; feed it company and the wall sprouts flight while never surrendering its ground-blocking duty. Unsupported, it holds the fort for a long stretch as a stationary 0/4, waiting for something else to finish the run.