I think it's "reunited exes" like yk Second chance love, like they are an established long-term couple that lost the spark on their relationship and trying to find a way back to each other
And aslo "Established couples" Plots exploring characters who are already together this one shifts the focus away from the high-school lovelife or any where they first fall in love to onto their adult life , living together, communication, domestic life
ugh yessss your mind. https://www.mangago.me/read-manga/oneness/ you might like this. It isnt full Lovecraftian horror but once he fully transforms it gives me those vibes
yessss!! there are a couple manga I have with that trope that I think you'd like. I have to search through my books to find them cuz I cant remember the name lol. When I get the energy to get out of bed, I'll reply again lmaoooo.
i also think there is a manga called ripples in the river or something like that where they are an established couple and the volumes go back to when they meet and all that. Havent read it but from what I've seen it seems really sweet
if u like webnovels there's someone on novelupdates that has a list for this
https://www.novelupdates.com/viewlist/94273/
i cant guarantee the stories on there r good (other than the second one on the list which is absolute peakism but u get it, u get it)

What's the most underrated BL trope?