A Mummy, an example of an X-Class monster.
X-Class Yokai are a very rare form of Yokai, that consist of only a few forms. Monsters of this rank are considered to be the strongest form of Yokai in the world, some even exceeding S-Class monsters, including Kitsunes and Werewolves.
There are very few X-Class Yokai, with only seven being known.
Forms
Mummies - undead who have to sustain a form of curse, and they gain immortality, and powers of the desert.
Wraiths - elemental human souls that wander the earth until they are to be judged in the afterlife.
Lich - humans who sold their soul to demonic forces, turning them into skeletal monstrosities with sinister, evil powers.
Titan- elemental giants who possess immense physical and magical powers, and are only capable of being killed by another Titan, or a human.
Rainbow Serpents - large, divine, reptillian creatures, who can control the forces of rain, and storms.
Nightwalker - formless demons, which often resemble large humanoid beings made out of pure darkness, with the power to drain life simply by touching another creature.
Dragons - A dragon is usually an elemental creature, typically with serpentine or reptilian traits resembling a dinosaur with wings, capable of using magic and usually breathe their element.
Becoming X-Class
In order for a Yokai species to become X-Class, a member of a one kind of species must defeat another member in a fair fight in a battle of powers. If one defeats an X-Class Yokai in such a fight, then that species will be considered to be among the X-Class beasts.
FanFiction
Rosario + Vampire: Redemption
Gallery of X-Class Monsters
