With nothing left for him in Hyrule and no future among the Kokiri, the former Hero of Time searches for an old friend. (Post-OOT, pre- and post-MM.)
A human falls into the Underground and faces the final boss: Queen Alphys. There's only one way this can end.
Freed from the Fresh parasite's control, Sans finds himself in an Underground that isn't quite his own. He takes what's been given to him as best he can. (Or: a slightly gentler alternate ending for a character with a disturbing fate, as written by a canon purist with no affection for AU content. With apologies to Hiroyuki Owaku.)
The Hero of Time falls. Hyrule goes on without him. The legend continues, as it must. (Based upon the Downfall ending and timeline between OOT and ALTTP. Follow-up chapter coming whenever.)