Apoptosis

Ilias: "It's called the apoptosis effect, a mutation phenomenon caused by unordered space-time itself. It acts as an immune system of this twisted spacetime, so to speak. It mutates organisms from its environment to repel foreign invaders."

Ilias, at the 1st Tartarus

Apoptosis is an immune mechanism phenomenon of twisted dimensions which appears in Monster Girl Quest: Paradox. It morphs life forms into strange creatures to slow the spread of Chaos by removing trespassers from the dimensions, thus preventing inter-world interactions, which increase the spread of Chaos. But this is pointless, as the Chaos spreads too quickly for the apoptosis to eradicate it.

According to Ilias, the creation of apoptosis based lifeforms and objects is the combination of holy and dark energy in a distorted spacetime. Think of it like Ilias taking the White Rabbit serum, but on a minor, and manageable scale, with the addition of some machine parts on occasion, which are integrated into the body of the life form. Where Chaos has spread, Apoptosis will follow, and corrupt the organisms and objects into the Apoptosis beings Luka sees in the Tartarus.

Also according to Ilias, once they are corrupted, they fall under the control of the will of space itself. But they do retain some free will, and can join Luka, likely because Luka shares a common goal like they do; to stop the spread of chaos. But this doesn't stop them attacking him upon meeting them, and are much harder to recruit than normal monsters.

According to young Promestein`s research, there are some apoptosis creatures that have similar or near identical appearances, behaviour, and even memories (Luxuru, for example). It seems like each type of apoptosis creatures do retain the face and body structure of the individual who turned into an Apoptosis creature, but Apoptosis has morphed them to accommodate for combat better.

Plus, according to Luxuru's quotes, only females are able to become an apoptosis creature, while males just turn into dust.

Think of Apoptosis as the white blood cells, trying to kill the virus. Chaos is the virus, spreading throughout the body. But the virus is spreading too quickly for the white blood cells to do anything about it, so it needs a cure, in the form of Luka. And the host of the virus is everything in existence, which it will eventually consume.

High-class Apoptosis creatures
Sonya

Adramelech

Low-class Apoptosis creatures
Sully

Jaide

Shesta

Eater

Riot

Luxuru

Vitae

Vetala

Valto

Shinifa

Amphil

Libo-Libo

Jellaila

Twinkell

Trivia

 * In biological terms, apoptosis is a biological phenomenon known as programmed cell death. It is essentially the suicide of body cells which are no longer needed or ones which may pose a threat to the body, and then removes any trace of their existence. Although, it can kill off the healthy cells unintentionally and the unhealthy ones stay by sending out the wrong signals at times. You can probably see a subtle link between Paradox Apoptosis and actual apoptosis. Paradox apoptosis transforms living beings into ones to combat trespassers, whereas biological apoptosis kills off unhealthy cells. But they're both naturally occurring phenomenon, and both are meant to minimise damage to the system, but it doesn't always work.