In the middle of a 3-man Vanaduke fight, he simply vanished whole, leaving the arena locked with three slag walkers still walking around.

He was in the middle of his second body phase, we had him frozen in about the middle of the arena with a shivermist buster, and were tossing water at him to put out his fireballs.

One of us may or may not have hit him with water at the moment he disappeared, but he had just started charging while still snared in ice when he did vanish.

Also of note, a party member reported that one of Vanaduke's slag walkers had also disappeared in a similar manner earlier in the fight.

We requested GM assistance at this point, and they were unable to determine Vanaduke's whereabouts. We attacked the remaining three slag walkers, and the final one also vanished as we attacked it.

The GM got us to a fresh Vanaduke fight that we were then able to complete without incident.