At the end of it, the players decided to invoke Ao, of all beings, and demand he assign anyone he could the role of Psychopomp. As a result, Ao, being the kind of overdiety he is, issued overdivine punishment as he so often does. He unwrote the events of his invocation and made the players replay the events from right before and choose a different path.
Ao began waiting on the sidelines to see what happened next before deciding his punishment of the mortals. Erasure, damnation, etc.
Then, the players made use of necromantic research they found to return only the storybook soul piece and help the Lich go peacefully. Had they just given all of the corrupt soul pieces back carelessly, the Lich would have gone berserk and an overtuned fight awaited them (full resources, but Deadly+N where N is a lot). They were smart enough to make a sound call, so they were supposed to get a good ending.
But they invoked Ao and he's a jerk.
So, Ao decided to unwrite all their accomplishments, those of the Lich, the players, and the Demigod Swordsman. They were cast back in time with no memory of the events of the session, to the early days of their adventures. But something was different. There were two new members to their party. I described the scene as a knock on the door, with the Great Swordsman coming in saying "Sorry, we're late," and a slow pan to another person holding a book at their waist.
On the cover, a depiction of a hero facing off against evil.
Ao isn't that big of a dick.
Highlight of Ao interacting with the party:
Player: "If no one is ushering souls that's really bad!"
Ao: "That is not of your concern."
Other Player: "I'm feeling pretty concerned."
Ao: "AS YOU SHOULD BE!"
Everyone: "What?"
One regret: Not realizing that everyone was going to be doing Radiant damage when I planned the bunch of Core Spawn Worms. On Round 1, after the first player obliterated themselves to deal as much damage to the largest one as they could, they had a good idea how the mechanic worked, and wanted to Chain Reflect radiant damage at all of them while the whole party was within range.
The one-shot almost ended Encounter 2, Round 1.
Oh, one last thing that came up in that one-shot that I think is really cool to do. You've all heard about the "How do you want to do this?" When the player obliterated themselves, I asked him "How do you want to go?" and he said "I leave no trace behind as I burn up."
I really like letting players decide what happens to their characters when they "die", but having that moment in combat where everyone is absolutely shocked and the player describes it, that's a new feeling we all got to experience last night
Wall of text over. Just thought I'd share as much as I could of that Level 20 one-shot. Everyone was calling me crazy for even trying to run something that high level, but the players and I really enjoyed it.