This is the beholder quest located in the lowerdark caverns, the issue being that once completed we are unable to end the quest but we don't know why.
Might be the quest not updating when the boss is slain.
This has happened to me before as well, in almost the exact same set of circumstances - boss slain, left, couldn't finish the quest and so we had to abandon it. I'll note that we didn't investigate the REDACTED
REDACTED