If you are stuck on Aven and it ask you to do Return Trip quest. Just stand near by and left the NPC talk. I think the NPC said He was like this when we found him. Then you click on Aven and a bar pop up. Talk to Lenora and you are done.

look that
https://infolao.tistory.com/entry/Rohendel-Skill-Potion

I thought I had to do the Phantom Palace in the Abyssal Dungeons. You actually have to do the Phantom Palace dungeon in Rohendel > Elzowin's Shade.

You have to do the Phantom Palace "map" Dungeon and kill the Phantom Legion Queen ( wich is the lady that spawn waves that you have to avoid by using the mirrors) once you kill her the first time you can already get out of the dungeon.

I did the dungeon on hard mode and only had to kill her once and was the "mirror" phase.

the link by Faja where it says you have to "kill the boss 3 times" more than likely a bad translation from (korean?)there are 3"bosses" (the rook ,knight nd Queen) thats likely whats meant by that

Note that if you have used Feiton powerpass then this quest is completed. Meaning it's pretty useless to do this quest again as you don't get additional virtue stats or skill potion.