I really wonder if Cylva is going to have anything to do here. It would be amazing to get her as like an optional story trust for the final dungeon that's available if you've completed the role quests. It feels like she's so intrinsically tied to what's going on, with Ardbert, but also with Unukalhai and by extension Elidibus.

Alternative possibility - the mystery trial is the Shadowkeeper. I have no idea how they'd work that logistically for people who haven't gotten to Cyella's revelation, but, dammit, it would be awesome.