[Olorin]

It could also be a problem with loading an animation. I've often had problems with Trolls not falling down when in "near death" because I was hitting them too rapidly. It seems that the 'taking damage' animation is set to override other animations of a character model. So hitting the troll so often that a new 'damage' animation was triggering before the previous one had finished means that the troll never got to a point where the lower priority, 'falling to near death'animation could get going.

If you've got a number of party members all hitting with multiple attacks per round, you may want to pull most of them off the attack when you get into the "near death" condition. This often works for me with trolls. I either order everyone to stop, or attack a different target, and the troll drops to the ground, where I can have one person attack with fire or acid to finish it.

I don't know if there's some kind of special animation that kicks in for Irenicus during the hell trial, but if reloading didn't fix it, give this a try.