It's been bothering me that I can't remember the name of this manga, so I finally broke down and decided to ask you guys.

I read it probably some time between 2010-2013.

It was on of those stories where the protagonist is recruited into a secret group to fight people from another world/realm.
I believe they may have been shadow or hell themed, but not too sure.
All I concretely remember is that one of the cast of build up villains was killed, and then resurrected by the big bad villian when the protagonist's group enter the world the villains originally came from.
I believe his true form was a beast of some kind.

The story wasn't particularly long running, and it was a grim themed story (although not in a way I found personally convincing.)

If anyone has any idea which manga might be, you'll silence the nagging tick that's been in my brain for years.