Duriel is bugged - first time got into a party it heals constantly and never dies.
One of his mechanics is a frontal cone where if you get hit by it he goes invincible for a few seconds while he grabs and eats the player character.
I got hit by the grab and this occurred and he basically kept his invincibility it looked like even after the move was over. I was the leader in a 4 man party at the time with my girlfriend also in the group. After trying everything we could think to progress the fight I finally left the party and low and behold his hp started going down again and he went back to doing his mechanics. I rejoined my gf’s party and was able to get back into the fight to get the kill.
Was able to recreate this several times so I think it’s all related to someone getting hit by the grab attack. That player needs to leave the party for the fight to progress.
reported by several players already