A century ago, the Vampire King Duzell was defeated by King Phelios, who also died in the battle. However, Duzell placed a curse on Phelios, and promised that his reincarnation would destroy the reincarnation of his enemy in the future. Now, Duzell has been reborn in the body of a feline kyawl and finds himself in the company of Lady Ishtar, a descendant of King Phelios. Together, the two decide to find Duzell’s long time foe and set forth on a grand adventure of love, tournaments, and laughter.
Both series are shoujo manga in which the main male character is a supernatural being (a vampire in Vampire Game and a demon in Mugen Spiral) who gets trapped in (or reincarnated in the case of Vampire Game) a form of a cat/cat-like creature. Also, both manga have a strong and/or fiesty female lead character. Despite having different plots, both series share a similar feel to each other. While neither series is technically a shounen-ai series (after all, they both have a main female character), they both contain mild shounen-ai hints/senarios/sub-plots (more so in Vampire Game). It should be noted that Vampire Game is a much longer series than Mugen Spiral. Still, if you like one, you will probably like the other.