This is detail, but if you let Thrommel died, steal Fragarach from him and give him a flaming longsword +1 instead(hehe chaotic good) upon raising him from the dead, in the end it is a glitch and it still say he is dead.
I was expecting either a half between death and glory considering the awfull penalty of been raise from the dead and losing such an artifact, or the plain full "The Prince is back and alive" message, mostly depending on how ressourcefull and powerfull the NPC is, and Thrommel certainly appear ressourcefull and powerfull enought to make it back and help his country just fine with a flaming longsword +1 instead of that artefact...hehe, anyway, it is detail, no more.
Of course, I was not expecting any reward since once back home, with divination, he would understand what I did and either retaliate or forgive depending on his "understanding"of the priority to fight greater evil...will the lawfull try to make the chaotic thief that did it because the thief was sure it was the best way for good to prevail(maybe not the only, but the most secure, which was all that the thief care for when good for thief pockets as well as "best" to prevail over evil...hehe).
(I would not have destroy Iuz physical manifestation without that weapon, and the thief honestly think that we would not have prevail on Zuttmoy either, yet, unsure on that last part) hehe
Not to mention that now that evil is defeated, althought I have no desire to sell that sword, I would honestly choose that path if the buyer would be Thrommel himself and kinda upset, would it be cheap or a rip off, unsure, it depend on how the stuff happen. Hehe...around such overwhelmingly powerfull item, they could have put more meat on the bone.