Just to clarify about the weird incest reviews, it's been over 27 volumes and THERE HAS BEEN NO INCEST!!!
This is one of my favorite mangas, which is no surprise as it's by my favorite mangaka, Rieko Yoshihara (same author of Ai no Kusabi). The story, art, characters, and world-building are so well done. If you're interested in modern world stories that have divine gods, warriors, spirits, supernatural powers, monsters, destiny, reincarnations, parallel dimensions, etc., and yaoi (definitely goes beyond shounen ai), then I definitely recommend this.
Trigger warning: Please note that this does contain r*pe. And not the glorified type. Kouichi goes through a traumatic experience that doesn't quite leave him even if he does recover and is able to move on from it. It also contains on-screen deaths, along with a bit of gore (not too much, but there are bloody scenes, though nothing extremely graphic).
Although the main couple is established fairly early in the story, this does contain some harem-y aspects centering around Kouichi. For people not into harems and might be worried this could become one, please still give this a try. It's been volume 28 and the main couple never stray from each other and no one cheats on the other, not even emotionally.
Other thing to note is that this is still ongoing. The manga started in the 1990s (you'll notice some of the cellphones are outdated, but it's not glaringly obvious besides that) and the original version isn't even completed. The current translations are only a few chapters away from being caught up with the current available chapters.