It is hard to see His form, there is a wavering about it to every sense including magic, as waves of heat cause air to waver but this is only Power. What is hidden is perhaps humanoid, perhaps horned. The wavering distorts all sense of distance, Dispater could be the size of a normal man sitting on a normal-sized throne, or He could be as vast as a mountain range seated on a continent and this throne room only a window onto it.
In Hell the most dangerous thing speaks first, and there is no question at all Who that is.
"Carissa Sevar," says a most courteous and gentlemanly voice, perhaps from there in the room, perhaps from a thousand miles distant. "You bring with you some company that is not entirely welcome in Hell, but I suppose you had no choice about the matter. Was your journey here a pleasant one?"