The yellow skull key is placed in the middle one of nine recesses in a wall. Open the southernmost part (with the lantern) of this wall to reveal a switch. Pushing it opens a passage in the northernmost part of the wall. In there, you find two more rocket boxes and another rocket launcher (sector 1157).
Using the red and yellow switches opens three sets of doors. Enter the one in the south of the switches where the arachnotrons came from, ascend the stairs in the northeast of this room, kill the mancubus and the revenant and walk behind the pillar where the imps came from. Push the recess in the green lights on the pillar's eastern side to open an alcove (sector 1165) with a soul sphere in it in the northeast of the room.
Of all the maps to occupy the eleventh map-slot of their WADs in the Plutonia series, "Will You Be My NME?" is the only MAP11 to contain creatures other than arch-viles as enemies. It is also the only MAP11 that does not potentially force the player to begin MAP12 with extremely low health by ending in a Type-11 deathtrap (the trap lies on the right-side exit path in Hunted, and in Arch-Violence, the trap is entirely unavoidable).