In the year 1999, Tokyo has become a battleground as the Seven Dragons of Earth and the Seven Dragons of Heaven fight to determine the fate of the world. The one with the power to decide which way the battle will go is a youth named Kamui Shirou. Will he join the Dragons of Earth and preserve the planet by ending human life, or become a Dragon of Heaven and protect humanity from ruin? The fate of all mankind lies on Kamui's unwilling shoulders as both sides clash to gain dominance. The Dragons on both sides are individuals gifted with superpowers and fated to participate in this final battle, but what end will Kamui choose?
Zen is a man with no past; he can't remember anything except for intense urges to kill, steal, and be a menace to society, making waves wherever he goes. After kidnapping the daughter of a general, Zen finds himself embroiled in a conflict between citizens of Amata and Galay, leading him to question his past once more. Together, Zen travels with a cowardly doctor to discover the truth behind his murderous ways...
While Blank Slate is only a two-volume series, it relates to X/1999 in the ways that they're both considdered dark, violent, and confusingly mysterious and suspensful. I rather enjoyed both series, but not all X fans will like Blank Slate. However, I still reccomend you check it out.
The land of the dead is a heavily bureaucratic place full of paperwork, and it is the job of the shinigami (death gods) to ensure that the dead stay dead and in their assigned locations. Tsuzuki Asato is one of the longest serving shinigami, but is somewhat flaky and difficult to work with. He is assigned a new partner, a rookie shinigami named Kurosaki Hisoka, who seems to be the complete opposite of Tsuzuki in personality. As the two begin investigating various cases they become entangled in the plots of the mysterious Dr. Muraki Kazutaka who seems to have something to do with Hisoka's past. Life is never dull in the afterlife as Tsuzuki and Hisoka deal with Muraki's antics, their growing relationship, and Tzusuki's own dark, mysterious past that continues to haunt him.
Both of the series involve an end of the world of some sort and people fighting against it. Despite numerous action scenes x/1999 and Record of Fallen Vampire are still deeply emotional. What I personally like about both of the series is that there are no real "evil" characters. Each character has his/her own background, personal tragedies and reason for the path he takes. If you liked one, you will like the other.