Okay so Sess and Kag I think have gone through time together, pretty sure this was a slow burn.

Years into the future, they own a hot spring resort or retreat thing. Sess has Kag in the dark about a “special” guest arriving until I think she hears a phone call? Not sure how, but she tries to figure out who it is. The special guest arrives and it’s and old man with I think either his daughter or granddaughter, he says something that makes Kag realize that this old man is Souta

This is literally all I can remember so I hope it’s enough to spur someone else’s memory!

Maybe , Immortals by Dani ?

EDIT BY WICCAN: This story is rated M,NO YIMs, and can be found here: http://www.dokuga.com/fanfiction/story/11030/1.