Y shows up here shortly. Plus the series already mentions the world of graymark passively. I won't spoil you more than that here. The synopsis is pinned in my discord's raw-spoilers channel.
This is literally like the worst nightmare. You are the sole person in a city thats sane and everyone else is fucked in the head by some devilish otherworldly creatures of the nether realm
Even if it hasn't finished being scantalated yet, there's some pretty obvious connections to Burning effect and some speculations
The psychic girl is Y, we see Y right after the disaster that destroyed humanity in burning effect and she looks exactly like the Psychic girl. We also have the passing down of the old guy's knowledge to her. She probably gets impregnated by an alien (when she got captured probably) and gave birth to Bear
The promised day is probably the return of the alien, Y tells Bear that he has to go fight the aliens and Y has stated that his/her goal is humanities survival.
The MC is probably Kai's ancestor since he's referred to as the king by the aliens
This is a pretty good story and a much better horror story than most.
As for the sexual assault tag, its not the really bad kind but its not handled particularly tastefully