I was playing a Shaman Totem build. My Storm Totem killed campaign Lagon (clear full HP bar) during Phase 1 and then my character was still teleported to Phase 2 Arena. I killed those tentacles and my character was not teleported back to main Arena. Lagon big tentacles blocking both left and right staircase and I was not able to walk back and talk to Lagon to progress the quest.
This situation happened twice. On my 3rd attempt, I purposely lower my damage output by not buffing my Totem with other skills, leaving Lagon some HP left before going to Phase 2 Arena and finally everything was normal and my character can continue progress the quest.
I forgot to take screenshot. Hopefully EHG can still fix it. Thanks.
Edit: I was playing Full Offline without access to bug reporting tool