1 Mike: This stone block rising from the floor is the crypt of Acererak! The most dangerous room in the Tomb! Do not touch anything!
2 Will: Hi, guys!
2 All: Will!!!
3 Will: You sound disappointed.
4 Dustin: Yeah, I was bracing for a death trap or to be annihilated by Acererak, and all we got was you.

The background in the final three panels is again from the Tomb of Horrors illustration booklet. This is indeed the crypt of Acererak. And you can see Acererak himself in panel 2: the light coloured skull resting on the altar behind Will, between Mike and Lucas. The dust visible in panel 3 is also part of Acererak's remains.

Although just a skull and bone dust... well.... Let's just say that he is extremely far from defenceless. The treasure on the floor here is real and valuable, but good luck to any adventurers who try to get out of here alive with it.