Unfortunately, the return trip for the avatar and the Pit Fiend will take another day, at the least... during which time, the messenger Draiss sent to Dispater returns, bearing a much more formal request, to attend the Lord of Dis in his tower. The wording is ambiguous, but, given the context, he can likely figure out exactly what it means; Tiamat herself wants to meet with him, with Dispater as a mediator, to keep them from attempting to kill one another.