Well, yaoi or shounen-ai, i'm not sure =_= I don't remember much just that the main character was having weird nightmares. Not about the past or anything but just weird nightmares that frightened him. I also remember that there was this other long-haired guy which he didn't have good feelings towards. At some point, the main guy turned into some sort of Demon and the long hair guy calmed him down and seemed to know what was happening. I believe the main guy had a brother but I'm not sure if it was a brother or a close friend.

I know the details are vague but would anyone care to help? Any help would be appreciated! ^_^

Maybe Crimson Spell
Sounds absolutely like it at least '_'

Unfortunately, It's not Crimson Spell >< But thank you for the help! ^^

Then, maybe Wild Rose or Wild Fangs both by Yamagishi Hokuto?

Sekai wa minikuidesu..shika shi! Yūjō, yūki, tsuyo-sa wa ikinokoranakereba narimasen..Tiruendo!

Not those either. Thanks for trying though! =)

Could it be Uragiri wa Boku no Namae o Shitteiru ?

You can, you should, and if you’re brave enough to start, you will.

Maybe Monochrome Factor? Not too sure also. Btw Monochrome is not yaoi, so it might not be it.

Akuma no Himitsu?

Maybe Zeus?

Perhaps Sleeper?

Koyoi wa Kimi to Chi no Kiss o?