What is SES-01?
Remember your first game session? All of the excitement and adventure, with dozens of mistakes and missteps? Yeah, of course you do.
It's X years later. Your players have returned to the place where it all began to relive their early days, to see what has become of that place and the inhabitants. Everything is exactly as they remember it. Nothing has changed in all this time... except one thing. The denizens of where ever are somber, saddened by an incredible loss. And they need the Party to help them.
Help them with what?
Bury the dead. There are x numbers of caskets in need of pallbearers and escorts. There are graves to be filled and prayed over. One for each brave party member that participated in Session-01. They didn't make it. The locals are not so much holding out for heroes, but burying their past. They are honored, loved and respected, but gone. So much tragedy.
What happened? This is all on you and your players to discover when you go back and play SES-01.