I wish I could help... following (๑•ㅂ•)و✧
Maybe this one?
http://www.mangago.me/read-manga/hengoku_no_calendula/
No if I'm correct the manga ended
But later the uke dissappers and forgets his memory about everything
Sorry I couldn't help. Any other manga similar to your description, that is known to me is
http://www.mangago.me/read-manga/jigoku_meguri/
It's ok because sooner or late I will find it hopefully. Because I am in no rush
I found it!
It was called Neo Arcadia - Yami Ni Saku Niji.
This yaoi manga is about a underworld ruler who falls in love with the son of something good but i forgot the title. The seme is the underworld ruler and the uke is the son.