A BL about a Hero trying to to save the world but he turns out to be a dragon?!?!?

I read this a long time ago and it stuck with me, but I forgot what it's called and I didn't bookmark it????

So the story goes like your usual shounen where there's a party that goes to slay evil. Thing is this black haired man turns out to be a reincarnation of the dragon lord?? and he can shift consciousness with previous dragon lords. His partner has issues with self worth and later in the story becomes the king and thus their wolrds unifies because they too, become one (lol).

The story largely revolves around the dragon boy's dragon tradition in which he should be fighting other dragons to be worthy of being the dragon lord, and his partner's journey to redefines dragons in humans' eyes (or something like that).

I'm pretty sure it's a shounen ai and not yaoi. This manga is suuuuper old though, so maybe it's hard to find someone that has read it, but I beg for your help!
I have heard that this story is actually a novel, but was adapted to a manga. Neither of them were ever licensed to English, so they're buried gems for intl fans :(