Legions of the Reglets

Bathed in sunlight, Aaroniero's real head shatters the Kidō binding him, and he needles Rukia for using Byakuya-caliber spells he never knew she possessed, noting she might have wounded him had she aimed the blast his way. She answers that unmasking him mattered more, and steadies herself to cut him down. He simply steps into shadow with Sonído, reforms Kaien's face over his capsule, and grumbles about Aizen raising a sun over a world where creatures like him need darkness.

He explains that he is the only Gillian among the Espada, admitted despite that lowly origin because his evolutionary potential is limitless. By devouring Metastacia's rotting body he absorbed its Spirit Body Fusion, and his tendril-mouthed left hand lets him take the powers and forms of any Hollow he eats, which is how he wears Kaien's genuine spiritual body and memories. As a parting display he releases Glotonería, a vast tentacled mass channeling every one of the 33,650 Hollows he has consumed at once. Grieving that killing Kaien delivered his body to the Hollows, Rukia can no longer lift her blade, and a thrust of Nejibana snaps Sode no Shirayuki and runs her through.

Aaroniero reveals himself as the only Gillian-class Espada, explaining that consuming Metastacia granted him Spirit Body Fusion along with Kaien's authentic body and memories. He releases his Resurrección, Glotonería, wielding the combined power of over thirty thousand devoured Hollows. Overwhelmed and wracked with guilt, Rukia is impaled when Nejibana breaks her sword.

Legions of the Reglets is the 267th chapter of volume 30, THERE IS NO HEART WITHOUT YOU, within the Hueco Mundo arc. Viz Media titled it Legions of the Regrets. It was adapted across episodes 155, 159, and 160, Testament, Your Heart is Right Here....