Crimson Spell

Prince Vald of the kingdom of Alswieth must take up a legendary sword in order to defend his people, but the sword is cursed: having taken it up and used it, Vald now transforms into a demon every night if he is not magically restrained. He goes on a journey to find a magician who can break the curse, and meets Halvir Hroptr, a powerful but arrogant mage who agrees to break the curse if Vald escorts him on his travels. So far, so boilerplate Heroic Fantasy, but on Vald's first night with Halvir, Halvir succumbs to curiosity and breaks the magical shackles restraining Vald's curse -- and decides that having sex with Vald in his demon form will both restrain the demon and feed his own magical power. The next morning, Vald wakes up refreshed, but with no memory of what happened the night before... except for some reason he can't stop staring at Halvir's mouth...

And so their journeys continue, with Vald and Halvir acquiring companions and developing a strange bond as they travel through various fantasylands and encounter demons, dragons, and quarrelsome magicians. Depending on where you're coming from, Crimson Spell is either a fantasy with lots of gay sex scenes or a Boys Love series with an unusually well-developed plot and setting; it's a genre hybrid that works better than most because it gives equal time to both elements.

Crimson Spell is licensed in English by Kitty Media with 2 volumes released. Tokyopop Germany is releasing it in German.

Tropes used in Crimson Spell include:

Alphabetical Theme Naming: The three named wizards from Celeasdile are named Halvir, Halceles, and Halrein.
- It is later stated that all wizards in Celeasdile have the "Hal" prefix.
Androcles' Lion: Vald gains the help of a dragon he'd previously offended (see "Our Dragons Are Different" below) by pulling several spears from its side to free it from a poacher's trap, taking serious injuries himself in the process.
Angst Nuke: Demon!Vald goes on an incredibly destructive rampage after Havi apparently dies.
Attractive Bent Gender: So it turns out that Vald is an expert crossdresser...
Badass Long Hair: Vald, when his transformations into the demon cause his hair to grow out. Mars also qualifies, though his badassery has thus far mostly been downplayed (see "Hidden Depths"), and Halvir is an extremely powerful and combat-capable sorcerer.
Boys Love
Berserk Button: If you touch Vald in front of Halvir bad things are going to happen in your vicinity. Most likely along the lines of fire and magical strangulation. God help you if he thinks he's dead...
- This also works in reverse. Don't hurt Havi, or Vald will turn into a demon and gut you.
Brainwashed and Crazy: Havi ends up like this after interfering with a demon's attempt to kidnap Vald.
Cast Full of Pretty Boys: And not a significant female character to date.
Chekhov's Gun: At the beginning of volume 2, it's revealed that Havi stole a number of valuable Spell Books from the Wizarding School when he and Vald left, and has read them in the interim ("it wasn't as informative as I thought"). The books are soon stolen and subsequently destroyed, but near the end of the volume Havi uses a spell from them to resuscitate himself from the brink of death.
Chronic Hero Syndrome: Vald, to the point that Halvir, Rimris, and a dragon have all gotten on his case about it.
Collector of the Strange: Havi.
Cooldown Hug: Rather more than a hug, in this case. Also played completely straight at one point when Vald's demon side starts emerging while he's defending a village from monsters, and Havi embraces him to calm him down.
Crazy Jealous Guy: Halvir towards Vald. See "Berserk Button."
- Demon!Vald is just as bad - if not worse - toward Havi. In the side story in which Vald and the demon switch personalities, he threatens to destroy the world if Havi leaves him.
Curse
Destructo-Nookie: Havi and demon!Vald manage to bend the bars of Vald's cell when Havi is brainwashed.
Deus Sex Machina
Don't Touch It, You Idiot!: is both the plot premise (nobody was meant to touch Vald's family sword), and a general failing of the cast, especially Havi, who can never leave well enough alone.
Dude, He's, Like, In A Coma! -- Given that Vald never remembers it afterwards and is apparently not in conscious control of himself, Halvir's nightly frolics with Vald-in-demon-form probably qualify as this. Ruruka certainly expresses such an opinion, and also chastises Vald for seeming a little too interested in Havi when he's unconscious.
Evil Weapon: Yug Verund. It's even red!
Explicit Content: The sex scenes just happen to be layered onto a really interesting premise... (honest!)
Fantastic Light Source: A fairy in a jar. (Vald sets the fairy free almost as soon as Havi's not looking.)
Generation Xerox: Halvir's teacher Halceles served Vald's ancestor King Fleivangr, who was also cursed by Yug Verund; while there has not been much detail yet on what exactly their relationship was like, Halceles was clearly very fond of him.
Hellish Horse: Havi's, named "Fang." It has clawed feet, More Teeth Than the Osmond Family, and it's omnivorous.
Hellish Pupils: Vald has these when in demon form.
Heroic BSOD: Vald flips his shit after he thinks he's killed Havi.
Heroic Fantasy
Hidden Depths: Mars, a vagabond and thief who claims to be a simple swordsman, takes on Gile, the sorcerer who brainwashed Havi and abducted Vald, off-panel while everyone else is dealing with demon!Vald's rampage. He wins handily, unscathed.
High-Class Glass: Havi briefly dons a monocle in chapter 28 while working in an official capacity at a checkpoint into an imperial capital.
"I Know You're in There Somewhere" Fight: Vald versus Brainwashed and Crazy Havi.
Important Haircut: Vald cuts his hair short when he leaves his kingdom to search for a means of breaking his curse.
Insufferable Genius: Halvir, to the point where the only person who can actually stand him is Vald. Everyone else merely tolerates him for Vald's sake.
Intimate Healing: When Halvir is poisoned, Vald passes the antidote to him mouth-to-mouth.
Jerkass: Halvir, whose affection for and loyalty to Vald is basically his only redeeming quality.
Lotus Eater Machine: In a side story. Havi being Havi, it takes him three months to get around to seriously investigating what's going on (even though he knows from the start that something's up). Good thing for that Year Inside, Hour Outside...
Lovable Rogue: Mars.
Love Redeems: While the ultimate resolution is as yet unclear, there are indications that Vald's love for Halvir is having a redeeming effect on the demon god.
Magic Knight: Demon!Vald is capable of using magic, albeit far more likely to just tear things apart with brute force.
Magnetic Hero: Vald keeps acquiring followers. Lampshaded by Rimris, who says it's because he inspires an urge to protect him.
Marked Change: The first sign that Vald is transforming into his demon form is the appearance of black markings on his skin.
Named Weapons: Yug Verund, the crimson sword.
Nice Job Breaking It, Hero: Many, many times. It can combine with a Don't Touch It, You Idiot! sentiment, or just reflect the huge amount of property damage that seems to reoccur around ~~Havi~~ ~~Vald~~ the protagonists. It would also be very helpful if Havi didn't have a way of angering most entities in existence.
Our Dragons Are Different: The first dragon they meet is starving to death and easily killed; the second is vast, and vastly powerful and refuses to help them because he can smell the death of the previous dragon on Vald. Vald doesn't help matters by declaring that he ate the previous dragon so as not to waste the body...
Perma-Stubble: Mars.
Personality Swap: In the Lotus Eater Machine side story, Vald and the demon switch personalities.
Pet the Dog: Havi's moments of kindness toward Vald help to soften his otherwise disagreeable character.
Power Gives You Wings: Vald sprouts little bat-wings when the demon's hold over him grows stronger, though they don't reappear in subsequent, less severe transformations. Later on, he temporarily manifests feathery white angel wings during a transformation brought on by his feelings for Havi; what this signifies remains to be seen.
Power Makes Your Hair Grow: Another side effect of Vald's more advanced transformation (along with the bat-wings mentioned above) is that his hair grows out. Afterwards he has to re-do his Important Haircut because of it.
Physical God: Ruruka is really close to this. Everyone else just doesn't realise it until he unleashes his true form and tries to fight Vald without killing him. ...Still doesn't stop Halvir from trying to kill him when he gets too close to Vald.
Rapunzel Hair: Halvir, and his teacher Halceles.
Really Seven Hundred Years Old: Halceles was around when Vald's sword was forged. That puts him at about 500+ years old, even excluding the decade or so he spent trapped by the dimension bug. He does not look it.
Royals Who Actually Do Something: Vald.
Running Gag: Characters lampshading the Shmuck Bait presented by the cuffs that keep Vald's demon side under control.
Sealed Good in a Can: Ruruka, trapped at the site of his previous master's death until freed by Vald. Also debatably Halceles, who was trapped along with Halvir's fellow students by the dimension insect until Havi came back to clean up his mess.
Sealed Inside a Person-Shaped Can: Vald is carrying a demon god around inside him. Good thing it's gotten attached to Havi's hot lovin'.
Spell My Name with an "S": There are surprisingly few examples despite the series gaining much of its following via fan scanlations prior to its licensed release. However, the official romanization of Ruruka's name, "Rulca," has not caught on with much of anyone (as demonstrated on this page).
Super-Powered Evil Side: Although interestingly enough, Vald's Super-Powered Evil Side - caused by his curse making him a channel for a demon god - seems more and more like he's being redeemed by Vald's love for Halvir
Sword and Sorcerer: Type 1. Vald can deal a huge amount of damage in his own right but give Havi enough time to work the incantation and he'll call up a creature from an alternate dimension to do his bidding. That or Kill It with Fire.
Tsundere: Type A, Havi toward Vald. Type B, Vald toward Havi.
Twin Threesome Fantasy: The Lotus Eater Machine side story ends this way, with Havi plus Vald and demon!Vald in separate bodies.
White-Haired Pretty Boy: Havi is powerful enough (and just about enough of a jackass) to qualify.
Wizarding School: Celeasdile.
Years Too Early: Vald, under the influence of the demon, informs the dragon he's fighting that it is "a hundred years too early."