Undertale AU by X2H
After a true pacifist ending, all monsters have made it to the surface; a year later, everyone seems to be doing just fine. Asgore, however, has become an outcast. After successful peace negotiations with the humans months prior, he has a hard time finding any further purpose, badly enough to where he gradually loses the will to live. He starts having terrifying hallucinations and nightmares regarding his past as his mind has had nothing to do but wander. Asgore tries to keep it hidden from everyone else and succeeds for a while, until Frisk starts to grow suspicious. One day, Asgore finally vanishes; only Frisk notices right away and manages to track him down.
Asgore makes his way back to the abandoned underground in a desperate search for closure, while Frisk follows at a distance. Flowey still lives there in solitude and is rather shocked to see Asgore return. Asgore reveals that he was finally able to put together