Quest or activity
Library Key is attached to a recorded quest outcome reward.
Recorded outcome: I found Radu in the abandoned wing of the almshouse – and killed him. He was working for Xanthe and the vrakhir thanked him for his service by turning Radu into some monstrosity. I delivered the grim news to the deacon – he was quite shaken, but still gave me access to the cathedral library.. The binding proves the result-scoped reward, not that every playthrough receives it.