He leaves permanently after completing Troubles in the Region quest.
Killing him results in reputation loss.
Speaking with Berrun completes the Speak to the Mayor of Nashkel task of the Important Events quest, and initiates the next task:
|“|| Visit the Nashkel Mines
Berrun Ghastkill has informed me about the situation at the Nashkel mines. I should make my way there to investigate the source of its problems. It lies to the southeast of the town, and I should be able to arrive there quickly.
Speaking with Berrun also adds another entry to the Troubles in the Region quest. If you spent any time gathering rumors in Beregost, you probably by now have a rather lengthy collection of entries for this quest.
|“|| Troubles in the Region
The mayor of Nashkel, Berrun Ghastkill, has asked for my help. I am to go to the mines southeast of Nashkel, and find out why the miners have been going missing. Something has also been tainting the ore, causing items made from it to be easily broken. Finding out what is down there is the first step in slowing the effects of the iron shortage.