If u decide to do this quest 'good way' u will end up with open quest in your log book so the solution to this is to use create undead spell on Brehgo corpse, he will then tell you that he will kill you slowly and painfully and he will attack you once u try unsummon him. After he is kiled quest is marked as solved on green and there is no more ghost (u can however sumon ghost to talk to) ps. sorry for my bad eng have fun fader
You can also go back to him and tell him that you lied about Arbalah lifting the curse. This will change the quest to the "botched" state.
What happens if you take the necromanced Brehgo to Arbalah? Don't have Arcanum installed at the moment (heresy, I know) so I can't check it out myself.
When I tried it in my game, nothing happened except that Arbalah ran away in terror when I released Brehgo near him.