The Vaizard

The Vizards are the antithesis to the Arrancar as Shinigami who have obtained Hollow powers.
Having begun as Shinigami, the Vizards appear entirely Human, and barring those times where they are using their powers are indistinguishable from ordinary Shinigami. To gain access to their Hollow powers, a Vizard dons a Hollow mask. This allows them to augment their Shinigami abilities with Hollow powers, as well as giving them access to some of the more generic Hollow abilities. Their spiritual pressure likewise changes into a mix of Hollow and Shinigami. This process of donning one's Hollow mask is referred to as Hollowfication.

The Vizards have been shown to be former Captain and Lieutenant Shinigami. They are considered criminals by the Soul Society for committing the ultimate taboo of acquiring Hollow powers; their status as former leaders of Soul Society also marks them as traitors

Process 1: This process is based entirely on force of will by a normal soul, though with the intention of gaining Shinigami powers it doesn't have access to or failing and becoming a hollow. Becoming a Vizard is a extremely rare occurrence of this process. When Ichigo's spirit chain was completely eroded he underwent the normal transformation of a plus into a hollow. The significant difference being that normally when a plus transforms into a hollow the spirit form shatters and reconstitutes itself into the new monstrous form. Ichigo's transformation was entirely out of sequence, as the hollow mask was being formed first while his body was still that of a plus, showing that he was resisting the transformation. Ichigo enters into an internal world where he is forced to find his spiritual powers in a small amount of time, by sheer coincidence he does so, but in the middle of becoming fully transformed into a hollow. Therefore it is that singular moment that allowed Ichigo to gain both Shinigami powers and acquire hollow powers at the same time.

Process 2: It is unclear how the other Vizards gained their current status. What is known is that Sōsuke Aizen was instrumental in the Hollowfication process (though whether he knew of the full potential results is left to question). This process used seems to spread as some form of possible infection through wounds, that has the desired effects on souls with high-level spiritual energy as compared to souls with low-level spiritual energy. Causing those infected great sickness and fatigue until they vomit up a white liquid that forms the beginnings of their hollow mask, which then proceeds to transform their bodies and warping their minds. Other than that all that is known is that the process is accelerated by aggressive emotions. The conclusive result makes a Shinigami into a primarily hollow-like humanoid which is a highly unstable transformation causing mindlessness aggression.

Process 3: Aside from the dangerous method used by Urahara on Ichigo, the Hōgyoku could allow a Shinigami to use Hollow powers (presumably in relation to the Vizard creation), but it has only been shown doing the opposite (creating the Arrancar). Urahara's Hōgyoku played a vital part in the process of making the Vizards currently what they are, as he tried to use it to return them to their previous state. But according to Urahara, what his device did was not what he had intended (turning them into a stable form of complete hybrids) and he considers the solution a failure on his part, though they didn't become complete Hollows due to his intervention and did eventually gain control over their new power.

Hollow Mask: The Vizard remains in complete control while wearing their masks, unlike Ichigo's early unstable transformations. As suggested by Aizen, the fusion of the two spiritual entities allows these Shinigami to reach levels of power far surpassing that of either Hollow or Shinigami.

The Vizards use their Hollow powers to augment their existing Shinigami abilities, granting them massive increases in speed, stamina, and strength.

The only known side effects to donning the Hollow masks are the specific time limits, which vary from user to user and can be extended with training, though to what extent is unknown. Every time the mask is used, the Vizard's voices echo as a consequence of both their normal voice and their inner Hollow's voice speaking at the same time.

Enhanced Speed
Enhanced Stamina
Enhanced Strength
Enhanced Spiritual Power

Reiryoku Absorption: Hollow absorb spiritual power (Reiryoku) from their surroundings or ingest it from spiritual beings, while Shinigami produce their own natural spiritual Power, some of which is then wasted, or leaked out. By combining these two properties the Vizards can reabsorb the spiritual power that they themselves give off, making their techniques much more efficient. They can also take it in from other sources, further boosting their power.

Cero: They also gain access to some Hollow abilities, such as the Cero blast,

Mask Shielding: Bodily harm can seemingly be ignored while wearing their masks, as demonstrated by Ichigo's Hollow during his unwilling transformations. The Hollow mask can only be maintained for a certain period before its effects wear off:

Power Augmentation: With the acquisition of the mask via the subjugation of ones inner Hollow. A Vizard gains access to a additional and separate source of power, in addition to the Shinigami powers one already possesses. The accumulation of both powers together greatly augments the powers of the Vizard beyond what a Shinigami and/or Hollow alone would be capable of.