Henkei

Henkei are described as entities that sunder the boundary between Arrancar and Shinigami or Vizards and Hollows, the perfect blurring of Hollows and Human Souls, creating extremely powerful entities that choose the allegiance of a new King rather than exist in the domain of the Soul King and the prejudices of Soul Society.

History
The Henkei were originally formed within the last century, the deepest wishes of Sōsuke Aizen causing the Hōgyoku - even while incomplete or unawakened, to begin concocting their creation in the depths of Hueco Mundo. The Hōgyoku gave Sosuke Aizen the capacity to create these new lifeforms, however they were incomplete by the time Aizen absorbed the Hōgyoku into his form.

Mistaken by most of Hollow kin for extremely powerful Vasto Lorde, the other Hollows soon learn to keep out of their way as they begin to awaken one after the other, over time it is revealed the true nature of the Henkei: a Curse that came about by the fall of the Fallen King.

First Class
The First Class Henkei are concluded initially to their King and his Five Royal Guard members, however it is later revealed that there are a minimum of Eight members of the First Class, each having the power to match or defeat multiple Captain-class Shinigami with ease. They are:


 * Soburin - The Fallen King, who was using Sosuke Aizen as a Vessel as early as the Thousand-Year Blood War and Yhwach's invite to Aizen to join his Special War Powers, Soburin's power is on a completely different level (some claim to equal the Soul King himself) to anyone faced so far and the seed of his rebirth was the Hōgyoku itself thus making the Hōgyoku's power having originally been his own.
 * Rozu - The Daughter of the Fallen King, officially she serves apparently as one of his Five Royal Guard members, but despite her great power her combat potential seems to be a lot less than expected of a Royal Guard, alluding to greater numbers of First Class Henkei than first implied.

Trivia

 * Henkei is a Japanese word that can be used to mean "deformation", "variation", "deformity", "modification", "transformation", "metamorphosis", "variety" and "monster".