Hollowfication.

Hollowfication;

During Urahara Kisuke's reign over the Technological Institute of Seireitei, a possible method of creating stronger Shinigami / abilities lead to researching combining a Shinigami's spiritual essence with that of a Hollow; integrating the two in a semblance of harmony.

the Betrayal;

After Urahara was promoted to Captain of the 12th Gotei, an overly ambitious Lieutenant (serving under the 5th Gotei Captain, Hirako Shinji) named Aizen Sosuke was already scheming against Seireitei.

Whether Aizen discovered and stole Urahara's research on integrating a Shinigami's essence with that of a Hollow and it's corresponding methods or came by this revelation on his own is unknown- but with the assistance of Kaname Tousen and Ichimaru Gin, he utilized it against a handful of Captain and Lieutenant.

Realizing Aizen's scheme, Urahara attempted to counter what he aptly named "Hollowfication" by confronting Aizen at the scheme of the abomination. Finding it prudent, Aizen retreated with his two subordinates while Urahara attempted to treat the victims of this heinous attack.

Though unable to reverse the process, Urahara assisted the newly altered Shinigami into a transition which unlocked new potential and kept their psyche in tact- but not without a price. Aizen had already been at work, framing Urahara for the "Hollowfication."

Immediately after apprehending Urahara, the Central 46 Court ruled that he would be stripped of his power as a Shinigami and exiled to Earth for eternity and that the monstrosities he allegedly created would be disposed of as Hollow; destroyed.

Having also come to realize the depth of Aizen's ambition, Yoruichi Shihouin made a break from Seireitei by rescuing Urahara and the unfortunate Shinigami- the Vaizard. Absconding to Earth, they all made new lives for themselves but didn't forget their grudge against Aizen Sosuke.

This was the first known stroke against Seireitei by Aizen Sosuke.

the Vaizard;

As Shinigami forcibly integrated with Hollow by method of "Hollowfication," the Vaizard lean more toward their original Shinigami psyche; retaining both their personality and a human appearance.

Though their lives were drastically changed, the Vaizard learned to cope with their new transformation and are capable of manifesting unique Hollow masks which greatly increase their overall power and amplify their abilities.

In lieu of this, Vaizard are the polar opposite of Arrancar.