MORE INFORMATION:
The Party: The party was composed of three members. Two of them hated each other deeply. After one ambush from their nemesis, one of the died but was immediately resurrected by a wish they had kept from a previous adventure. The party member didn't want to be resurrected (it had been his second death and he started to believe that people should not be resurrected after death). I DM that a wish is capable of bringing back the dead, regardless their permission.
Angry, the party member that was brought back decides to leave the party and the quest as he cannot forgive them for ignoring his wishes of staying dead.
The Quest: The party needs to heal a friend of theirs. The only healer capable of the magic needed to heal her gave them a quest in return. They need to fetch an artifact of her god from a certain dungeon. The party was still traveling to the dungeon when their nemesis ambushed them. The dungeon is a long forgotten and abandoned place.
The New Guy: The new guy is a worshiper of the Raven Queen. His goal in life is to (aside from following the commandments of the Raven Queen) is to find the missing family of his best friend (who died).
Sorry for triple post, but it seems like my suggestion, combined with Caliban's, would suite perfectly here.
They continue on, a man down, a wish lost, angry and discouraged, and run into an old friend, who is there on his own secret business. Roll Credits on surprised faces, party member saying, "Frank? What the f-"
Next session starts with flash back. When flash back resolves, cut back to just after the wtf moment. Be sure to tie the flashback adventure in to the reason he is there, for bonus points!
I just did this as part of relaunching my Eberron campaign, and to deal with new players joining the campaign after the first adventure had begun, which was
itself a framing story where two PC's were telling the story of how they met an DMPC to an old recurring ally and sometime DMPC, in a tavern in Sharn, on Rememberence day.
Now it's a flashback in a flashback, as one of the PCs recalls how she met the people who they've just run into, who are caught up in the Thelanis Manifest Zone effect that is playing out an old fairy tale near the Cyre border, at the same ruined tower where she met them 2 years before!
Soon, they will all meet again in the current year, at the Remembrance day celebration, and the mystery of the Prophecy Mark they found in the tower, and how it relates to they labyrinth and the treasure within that is the center of the Thelanis Manifest Zone story. They'll have little time to wonder at the intricacies of prophecy and destiny, though, as they chase doom through and over the streets and towers of Sharn!