Did you kill him at a place called Garzahd's Fortress in Fiery Region? If so, King Micah should tell you to read the 2 tablets in his room as a small token. The only place I can think of where Garzahd appears is at Ornotha Ziggurat. You can't and not supposed to kill him there.
If it is a bug, you can read the solution at the end post here: https://spiderwebforums.ipbhost.com/topic/22237-a2cs-questions-possible-spoilers/