This quest doesn't close properly if prior to taking this quest his soul was already taken as a Source point. If you go and destroy the jar later, quest log says that Withermoore is free and he lies dead but the quest doesn't close.
Similar to above. I freed his soul (by breaking jar) and quest not complete. And not out of order shenanigans either as I already had met Withermore and accepted the quest. Even went back to Withermore in the cave to find his body on the ground and the kid crying. It made me feel bad.
Has anyone tried putting his soul jar in the inventory and taking it back to him in the hideaway to see if that resolves it differently / marks it complete?