Abyss Levoire

Main-path story objective, so unmissable - the run through this dungeon is on the critical path. Worth knowing for planning though: coming back out of here is the big point of no return. Once you let Adam take you onward after this, the Xion side quests and collectibles lock for that save - so retrieve the cell, then finish everything in and around Xion (Beep!, Beyond Fate, Sisterly Love, Brute, Behemoth and any leftover collectibles) before you advance.