near the end of the boss fight against longtooth in the kurast undercity cave district, he leaves to send in minions and gets stuck in the entrance hallway to the arena (you can see the boss icon in the minimap) and never returns. You can see messages about his status… hunting, thrumming, drowning… but he never returns to the arena and the only way to continue is intentionally die and try again. I managed to kill him after about 5 tries (he was bugged in the same way in all but the last attempt)
i just lost my hc character to this, had no way to get out and he kept going for over 5 minutes
this is still bugged as of 10/23, but at least I was able to escape death in hardcore by using the Party Finder