Ryuki is a demon from Ashurado, the fourth level of Hell.
In 1864 he was invoked by Hana Yanowa when Hana fought her sister seeking control of the Shosei order. This allowed Ryuki to take control of Hana's body. Together, Ryuki and Hana would raise the dead seeking the Blade of Blood, a weapon which would give Ryuki the power to control the world. Ryuki was weakened by Wolverine when he coated his Adamantium claws in gold and sent back to Hell when Mana killed her sister.
Ryuki was created by Akira Yoshida and Shin Nagasawa in 2005 and first appeared in Wolverine: Soultaker # 1.
Ryuki has the ability to raise the bodies of the dead and control them to do his bidding. He can also absorb souls of those he defeats to grow in power.