Hiya! Kind of irritating bug, or I-assume-bug. Here's the things.
I cleared De'arnise keep, killing the troll boss and doing everything else, finishing with my visit to Glaicen for the third flail head. I used the RoHI on him to charm, and its success casts a dispel magic and opens a dialogue. He comments on his thankfulness that the troll boss is dead, and mentions that he must find Captain Arat. He then leaves, and you get your experience bonus.
Logically you'd find him with Captain Arat, but he is not there, and I cannot find him anywhere in the keep. If he is gone forever like I suspect he might be, then the third head of the flail is gone with him.
As a workaround, I was able to pickpocket the flail head off him before he disappears, but I figured I'd mention the bug anyhow.
For clarity, the process:
1) Kill TorGal of De'arnise keep
2) Charm or otherwise dispel magic on Glaicen after killing TorGal
Result : Glaicen will reward you with exp, mention that he must find Captain Arat / other survivors, and leave the area without providing you the flail head [acid] that he is carrying.
Expected result : Glaicen would provide some indication / means of acquiring the flail head [acid] either during the scene, or afterwards near Captain Arat, where he claims to be heading.
Note : It is possible to pickpocket Glaicen of the flail head before he leaves, provided you know he has it in the first place already.