Rusty key (Thronebreaker)

Rusty key is a key in.

Journal entry
''Fortune smiles upon us! Those priestesses bought my story, blubbering and all! They accepted me as a pupil, but getting me hands on the cellar key was the tricky part. Still I've gotten it now too. They've riches stacked to the ceiling! Gods be praised, indeed! Wait for me tomorrow night at the foot of the embankment at the south wall. I'll carry what I can manage.''

''P.S. Keep an eye out. Probably just a wives' tale, but I got word some sort of nastiness has taken root in the wood.''

Source
After completing the Behind the Wall battle in Lyria, loot the corpse to find the Rusty Key.