Suikei Ajari (the main uke) was the crown prince Atsuakira's twin, and that time twins were seen as misfortune so one of them must be killed when they were babies. But the man who supposed to kill him took him instead, raised him as a priest in Mt.Kouya and prepared him as a curse "vessel" one day, meaning he would be the one to take enemy's curses that were supposed to be directed at the Crown Prince, without the Crown prince knowing about his existence.

Ashiya Douman (the main Seme) was found by Suikei when he was a boy, wounded and deserted in the snow. As expected, he's head over heels towards Suikei. Later he also became a Priest in Mt.Kouya, and they had lived quite a peaceful life together until 'incidents' happened (what incident, it hasn't revealed completely) that he had to left the place. (My guess, one of the reasons was because he "tainted" Priest Suikei while to be a "vessel" Suikei has to be pure and such)

I make it sounds so simple and cliche but believe me it's not that easy to grasp the problem, because the timeline's random, and it's like broken pieces gathered together. The story began with Douman battling shits in the mountain with the cursed tattoo on his arm together with his freakshow convo, busy in thoughts, mumbling who-knows-what, and how they're still connected and shit. Flashbacks, Banshees and Elderly conversations take places randomly. It turned out that that day was the day where Suikei started life as "vessel", hence Douman wanted to get him out of where he was.