Chimera

A Chimera(, lit. Curse of the Blood) is a term used in Highschool DxD: Яe-birth continuity to refer to hybrids/aberration of several different races and factions across the different lores. Unlike the more known and conventional Hybrids however, those classified as Chimeras were artificially made using several arts of magic, alchemy and/or Divine powers, resulting in a abomination that shouldn't possibly exist in the realms of the current reality, even with the death of God from the Bible and the previous Four Demon Kings. The process in which oneself turns into a Chimera is called "Chimerization"(血の呪い(キメラ)化; Kimera-ka).

Although the most known types of Chimeras are reincarnated devils/angels with Sacred Gears, other processes such as forced Sacred Gear/Light element insertion and artificial implants from different races and factions are not unhear of.

Summary
Chimeras were first introduced by Mary Beelzebub to Ichijou Tsukino in Volume 8 in the Highschool DxD: Яe-birth continuity. According to the Demon Lord, differently from the usual hybrids, beings labelled as Chimeras are artificially created with magic, alchemy or other unconventional means in other for oneself to surpass the boundaries existent between races of conflicting natures, such as angels/devils, Gods/titans or even humans and different entities of the supernatural, making them stronger and more resistant to their own kin and natural weaknesses, respectively, not unlike Demonic Holy swords. She also points out that, for using items created by the original God from the bible, reincarnated angels and devils with sacred gears are the simpler examples of Chimeras and, as such stronger than their counterparts.

The existence of Chimeras, however, dated a few volumes earlier in the fanfic. The Black Team, specially Rei Miyama, for being experimented with several different genes of different factions plus forced Sacred Gear transplants, are another examples of unconventional Chimerization.

Evil Piece/Brave Saint Reincarnation
The most common way of chimerization, since only humans are capable of wielding Sacred Gears and holy/demonic swords, their reincarnation as either a devil or an angel, thus adquiring respectively the ability to cast spells and Light-based weapons, makes them superior to their normal, human-leveled counterparts. Other species who are reincarnated as devils/angels, such as valkyries, dragons and vampires, can also be considered chimeras.

Light Eaters
Light Eaters, also known as Dark Hollow, is another process of Chimerization exclusive for humans and devils, in which oneself of one of those races obtains special abilities and powers by devouring any holy creature, mainly Angels and Fallen angels. Lost in history, Light Eaters are growing more and more as the balance between the Three Factions and the human world grows thinner, as well as the boundaries between them and their powers becomes less noticeable. Created during the Great Apocalypse, all Light Eaters shares one trait: they have the name of their eaten fallen angel printed on their tongue, named Mark of sin(罪のマーク; Tsumi no māku), to remind them of the unspeakable atrocity they have commited, One of the most common abilties gained from it is an immunity against holy objects: demons can still sense, but no longer possess "devilish allergy" to them, as crosses, rosarios, passages from the bible, holy water and so on won't affect them. They also gain an extreme shades of luck, since the flesh they had consumed are holy, and humans will gain stronger auras to attract or intimidate other human beings, while advanced Light eaters can even materilize Light weapons like angels. Sacred Gear users can unlock Balance Breaker faster by turning into Light Eaters, however, such unnatural evolution may cause the Sacred Gear to break, killing it's host.

Devils guilty of such crime, unless they're powerful enough, are purged from devil society and killed if ever found.

Transplant
Only known as Transplant(移植; Ishoku), is a form in which under intense care and magic control, oneself can literally transplant a foreign organ or body part into it's own anatomy, such as devils with arms with angelic origin that can create light weapons or sentient beings which transplant organs of other creatures in order to adquire new special abilities such as fire breath or special Vision. Sacred Gears forcibly put into oneself is also a form of transplant. Sōsō's Medusa Eye transplant and Raynare's use of Twilight Healing are canon examples of a chimerization.

Grau Krönung
Grau Krönung, german for Grey Coronation, also known as Fusion of Light and Darkness, is a form of Chimerization exclusive for angels and fallen angels. In a process similar to vaccine, angels and fallen angels alike can inject oneself with small doses of blood or fluids of demonic origin and, with time and effort, be able to adquire immunity against demonic spells and demon swords, also adquiring an immense boost of strength and holy power. Their presence also reaches a realm beyond the three factions, and such, undetectable by either of them.

Fusion
The rarest type of Chimerization, Fusion(融合; Yuugo) is capable of creating completely new types of chimeras using preexistent ones and the rare ability to merge them in complete harmony. Unlike other forms of Chimerization in which oneself still retains some sense of identity about their original form, Fusion destroys any form of identity within the fused components and constructs, in complete harmony and care, a new form based on them. Some fusions are so ridiculously powerful, they become inperceptive for devils, angels and even Gods, since their alien existence cannot be comprehented even by higher beings. Demonic Holy Swords and Transcendant Swords such as Ix, which has demonic/dragonic/holy and dragon-slaying abilities, and Beowulf, the fusion of Hrunting and Nagelring, are considered Chimeras created by fusion due to being created from preexistent entities and objects, yet don't retain correlation with either. The angel Ophelia had the ability to fuse objects, and as such, could fuse Rhongomiant and Ascalon in order to create Ix.

Abilities
Chimeras can adquire several abilities with their chimerization, including powers which were originally impossible to one's race; higher senses, strength and speed; immunity; Sacred Gear powers; the ability to use holy or demonic swords, etc.

When a Chimerization reaches a certain point beyond anything, mostly through fusion, the chimera in question can reach the status as a trancendent Existence(超越的存在; Chōetsu-teki sonzai), and such they become completely undetectable to devils, angels and even Gods since their sole presence is extremely alien and abnormal for "normal" beings to comprehend. Mary describes such as "Two-dimensional beings unable to understand and feel a Three-dimensional existence".

Drawbacks
Since Chimerization means merge different, even completely opposing, forces, there are high risks of rejection and even death if proper and extremely meticulous care are not taken. A human-chimera might be unable to control the foreign entity and as such can succumb to it if not infected and killed. Same goes for Light Eaters and angels who gone through Grau Krönung, since the presence of their respective counterparts are originally poisonous to them.

Fusion seems to be the safest way of chimerization, but there's also the risk of lost of identity for the new being created.

Known Chimeras
"NOTE: Only chimeras of the Highschool DxD: Яe-birth continuity will be listed."

Evil Piece/Brave Saint Reincarnation

 * Ichijou Tsukino(Boosted Gear user former-human)
 * Nero Gladius(Role-praying Game user former Human)
 * Hwan Jin-Young(Former dragon)
 * Annabelle Hellsing(Former Dhampyr)
 * Michelle Isra(Holy-sword wielder former human/humunculus)

Light Eaters

 * Cohen Lucifer(Devil)
 * Black Team
 * Rei Miyama(Nephilim)
 * Kiruko Matsuoka(human)
 * Sarah Leonheart(Human)
 * Kurumi Kurisu(Human)
 * Eiko Onizuka(Human/Cyborg)
 * Suruga Miyama(Human)
 * Elizabeth Albedo

Transplant

 * Black Team(Originally human/Nephilim; transplanted with several other entities)

Fusion

 * Ix(Former Ascalon; Rhongomiant; Ddraig's, Yagura's and Ichijou's power)
 * Beowulf(Former Hrunting and Nagelring)
 * Michelle Isra(Former humans with light elements)

Trivia

 * The idea is slightly based on Hollowfication from the Bleach series.
 * The term Chimera is actually based on a scientific term in which a single organism is composed of cells from different zygotes. No correlation with the monster of same name.