Oscuro

Oscuro (執拗野獣, Spanish for "Dark", Japanese for "Relentless Beast") is a rare Arrancar ability utilized by only a handful of individuals. At a glance, it might be conceived of as a healing technique, because it heals it's victims wounds within seconds, being fit to restore an Arrancar to full strength in the span of a few moments. However, following it's recent emergence in Hueco Mundo, it's been revealed to be something else entirely. It requires training to learn, and it requires that the user possess the ability of beforehand.

Description
Most Hollows are capable of from the point of their creation, and it avails them excellently for as long as they possess it, but once they choose to become Arrancar, this ability is often discarded for greater power. For this reason, most Arrancars are forced to rely primarily on their skills and Hierro to survive. That is, until this technique was discovered by Arrancar Scientists under the command of. The principle of Oscuro is simple, for with it's use, the practitioner reaches into the recesses of an Arrancar's broken soul and forcibly imparts upon them high-speed regeneration. Causing their wounds to heal, bodies to reform and limbs to regrow at a great pace - because of the nature of Oscuro, it is not reliant upon a willing recipient, and it can be applied by force if necessary. After the regeneration is complete, they lose the ability until it's forcibly imparted upon them by Oscuro at a later date. The process of being healed by Oscuro is described as extremely uncomfortable, but immensely effective. Because Arrancar have naturally denser and more resilent bodies than ordinary souls, thanks to their past nature as Hollows, Oscuro inflicts no actual damage on their bodies and it's concidered completely safe in that regard.

Shinigami however, are much less fortunate. Much more fragile than Arrancars, Shinigami affected by Oscuro experience excruciating agony and find that their bodies are grotesquely deformed by the technique, even if there was no wounds to heal originally, direct contact typically results in large tumors forming both inside and outside of their body. The extent of damage this technique inflicts upon Shinigami being dependant on how long they made contact with it - a glancing blow might only result in minor discomfort, whereas continuous exposure can result in deforming the poor victim until they're little more than a bloated mass of flesh. Kaidō is highly ineffective against Oscuro, because the reiryoku left in their body repels them from healing, in fact, they typically only make matters worse by causing a conflict of energies, which result in tumors growing at alarming rates until they eventually pop, leaving large gaping wounds in their place. Because of the great suffering it inflicts on it's victims, most Shinigami who know of it have described it as disgustingly wicked.

Against humans, it usually inflicts various cellular diseases, usually various cancerous infections.