Their existence is negative. Negativities.

—Alice Hazuki

Negativities are a species of paradox monsters that appear when Rifts open.

They differ from Onyxes in the way that they are not biological and do not actually live inside rifts, but are the results of paradoxes caused inside the Akashic Domain, and as such can only escape in the real world through open rifts that they themselves create. They are almost invisible to the eye, have no soul or a specific physical appearance, and generate no magic. They only faintly appear under sunlight and UV rays as very faint yellow light sparks. Consequently, they are terribly hard to notice if one is not specifically looking for them, as only Mariya is able to detect them immediately. Furthermore, Negativities consume everything they get in contact with no matter what it is or what it is made of. Flesh is outright disassembled and matter absorbed then destroyed.

Keepers are tasked with erasing paradoxes to make sure Negativities are never born and if too late are charged to kill them due to how dangerous their existence is. The presence of a single one "fractures" the surrounding zone and tends to create more of them fast, often resulting in terrible consequences both for the people living nearby if there are any, and for the stability of the area. Actually murdering one doesn't require much talent as a simple magical spell will kill them but it still requires a lot of precautions.

Curiously though, despite their sort of "non-existence" they seem to display a macabre form of human sentience(?) upon death, as killing one will splatter the surroundings with an incredible amount of blood while releasing a loud, extremely ghoulish high-pitched scream. Studies on the blood from the Hellsing Labs have revealed that the blood is of unknown provenance but is always the same no matter what Negativity is killed. As such, the only valuable explanation is that the blood is generated on the very moment the kill happens, and that it is the result of a paradox that constantly reiterates itself every time one is murdered, implying that the blood does not belong to, or directly originate from them. [...]