This quest is actually called Find (Zero)!
After Completeing Underground Dragoon Ruins, return to the Ministry of Arcanum and Alesche will intercept you and ask you to find her. Return to Illfalo Port and speak with her, and she'll mention Zero chilling in this hot new dungeon, Descension Ruins. She asks you to consult your nooblet friends from the start of the game (Serge and Solgram, the NPCS from the opening cutscene)
After A quick conversation, you're off to DR! Before you can complete this mission you need to be in the process of unlocking the third final battle statue for the dungeon.
You'll eventually have to find a slate (6/6) for the statue. Make your way to the ressurection statue prior to the second sewers entrance. For those of you who remember Aria, it is the ressurection statue prior to the bull spawn final battle statue. From the statue, decline into the sewers and take your first left. Run past the frog and the couple of traps, and then duck under the low wall. Take your next right and follow it unti lyou can go left. Take that left until you find a stair well leading up out of the sewers on your right hand side.
From here, Take an immediate right and open the door. Make your way across the room and up the stairs. Take a left at the healing spring and then another left at the top of the stairs. You'll enter a room full of ghost mobs. The room will contain the slate (6/6), but also a ramp. Sprint-jump off of the ramp and across the ravine successfully, and you'll start the cutscene for the mission.
After the cutscene ends, turn the quest back in at Alesche.