Welcome to the Company of the Few, the Brave, and the Proud. (well, ok, the Many, the Brave, and the Proud.)
Welcome to the Company of all those who have died in the Tomb of Horrors.
If I were DMing, here's what I'd allow:
The Demilich is feasting on the souls, but they are not dead yet.
A high level NPC is going to attempt the Tomb. The players can bring new characters with him to help him.
If the party gains the crypt, the high level NPC will attempt to kill the demilich (but a player must run him in that battle.) If the demilich is killed, resurrection is possible for the original party. The high level NPC knew the original party.
On the other hand, if the demilich wins, chalk up another mass casualty to the most infamous of all the killer dungeons.
If you are a really, really kind DM, Acererak decides to release the souls, and he creates new bodies for them to inhabit.
But it comes at a price. Acererak keeps all of their items, and each character suffers a permanent Curse (one stat drops to 3, or 50/50 fail any saves, or 50/50 a hit on an opponent is a miss,, or 50/50 your spell hits someone in the party.)
Acererak has decided to let you live, as an example to everyone else. This is what happens to those who dare trespass upon his tomb.
Or perhaps Acererak is - being proud and cruel - merely having a good laugh at the party, hoping they will return for vengeance ... and then he WILL devour their souls.
EDIT:
My cleric, Edena (after whom I named this SN) was the lone survivor out of his whole party who entered the Tomb of Horrors. Edena survived because he was the weakest character in the party, so Acererak decided to kill him last. By the time Edena's turn to die was up, Acererak was so weakened Edena's attack dispelled him. Edena failed to take the prerequisite steps to permanently dispel him, though, so Acererak came back a week later, as strong as ever ...
I know of another party where the Cleric of Aphrodite was the only survivor out of a very large party.