I'm looking for a manga where a shape shifting dragon with time magic from another world entered the parallel world to kill magical creatures that ended up in the parallel world, with the end goal of sacrificing his life. He does meet up with a group and becomes "bonded" with one dude who I think was a holy knight or something like it. If it helps I think the boy was the dragon king.

Pretty sure this one isn't the one you're looking for, but Code: Leviathan is about a "dragon"-man that's transported to another world and bonds with a young boy (who becomes his rider / activator or something?).

Isnt that Yasashii Ryuu No Koroshikata? I really liked it too and wish that the scanlation hadnt stopped. ?