I was fighting Heorot for the 4th time in a corruption 100 mono, when he suddenly disappeared. I still took constant damage, and had 10 stacks of cold resistance reduction. While healing through it and running around like a headless chicken, I eventually portalled out. There was a white ping circle on the minimap, where I suspect Heorot should have been, as well as a small healthbar on screen somewhere, which could have been the around 30% health he had left.