Eidolon

"It may look like a person and talk like a person and maybe even bleed like a person, but it's a monster."

- Jace Wayland

An eidolon (plural: eidola or eidolons) is a race of shapeshifting daemon who can possess and transform into any human being.

Overview
In ancient Greek literature, an eidolon (Greek: εἴδωλον, "image, idol, double, apparition, phantom, ghost") is a spirit-image of a living or dead person; a shade or phantom look-alike of the human form.

The concept of Helen of Troy's eidolon was explored both by Homer and Euripides. However, where Homer uses the concept as a free-standing idea that gives Helen life after death, Euripides entangles it with the idea of kleos, one being the product of the other. Both Euripides and Stesichorus, in their respective works concerning the Trojan Horse, claim that Helen was never physically present in the city at all.

In Theosophy, the astral double or perispirit or kamarupa after death, before its disintegration, is identified with the eidolon.

Summary
The eidola, much like all other races of daemon, were the creations of the original demons who used them to wreak havoc in the mortal world, especially on Earth, and bring death and destruction upon humans. They, in particular, were the creations of the original Belial who created them to sow lies and mistrust into the hearts of men and to be harbinger of misfortune of whoever sees their faces.

After the original Belial was sealed within the Underworld with many other demons because of the Ouroboros Barrier, the Alpha Eidolon pledged allegiance to the Belial Clan of the 72 Pillars, true descendants of their creator, and thus, the true leaders of the eidolon daemon.

The current Lord Belial, Baelor Belial, however, disagreed with this notion and relinquished the majority of rulership back to the Alpha Eidolon, only agreeing to be the sub-leader of the race after much begging from his old friend and teacher.

Appearance
Normally, eidolon daemon in their human form are practically impossible to separate from regular human beings. However, due to them only "borrowing" the appearance of a pre-existing person, there will always be two versions of the same person who are similar from their personal preference to even their DNA, possibly even more if said person's appearance is taken by more than one eidolon.

In their true form, an eidolon daimon is a white, greasy and slimy like a giant slug, with their body covered in gray pockmarks. On films and cameras, their eyes will be a glowing silver-gold color.

Characteristics
Eidola are born as slug-like creatures that are "more slime than slug", but eventually grow into a humanoid form and later on, learn how to shapeshift their form to look like anyone they want. When exactly they learn is unknown, as some can instantly learn it from infancy, while others do not learn until their teenage years, adulthood, or even much later in their lives. As all eidola, excluding the Alpha Eidolon, are born from human mothers, they are born with human impulses and desires, and are not driven by an instinct like vampires and werewolves, and thus, all acts committed by them are purely their choice.

When an eidolon takes the shape of the person they choose, they shed their skin, teeth and nails. When they change into the person's physical appearance, they access the thoughts of the person they are mimicking. Some eidola are known to change their shape in a matter of seconds, while others require several hours. Some descended from the Alpha Eidolon have displayed the ability to seamlessly shift into whoever they want without shedding their skin. They can also take the forms of fictional characters or inhuman characteristics, such as vampire fangs or werewolf claws.

Despite this, not all eidola can perfectly take on the form of another human being, either as the result of their low amount of demonic power or their lack of experience and/or age. Because of this, these eidola would often be disguised as "generic humans" with unmemorable features, or members of a crowd, rather than impersonating specific individuals.

Some old and experienced eidola can also possess the bodies of human beings and take over their mind and actions with relative ease. To do so, they are required to be in their true form, which is their most defenseless state, before slipping into their intended host's body through different means, though the most common methods are through the ears, nostrils, or throat. In this form, they can be exorcised using the standard rituals and will leave their victim's body as a "dark wisp of smoke".

While some eidola appear to simply cycle through appearances and never settle on one particular permanent appearance, other eidola appear to have a preferred appearance they use when they are not impersonating another person. Often, this chosen form is what they would have looked like had they been a normal human born between their mother and the form their eidola father took, though some forms can also simply be the one they like.

Abilities
Demonic Power: As daemon, eidola possess all the standard powers and abilities of a common devil, including the power to cast spells, as well as their own unique demonic power, which they use to transform themselves into various different forms and appearances. Like other daimonic races, their demonic power increases extremely slowly and can only be speeded up with the consumption of human's life-force, especially in the form of their flesh and blood.
 * Shapeshifting: The most well-known ability of the eidola. They can take on a living or dead person's physical appearance by shedding their skin. When they shift, they literally become the person they're mimicking, gaining everything, including fingerprints and DNA; they can also mimic wounds and illnesses, even very serious ones. Once an eidolon transforms into that person, they can access the thoughts and memories of the person they are mimicking provided that the person is still alive.
 * Regeneration: They can heal nearly all non-fatal wounds afflicted upon their body, even replacing body parts such as ears or a big piece of skin. While posing as a human, they can endure the diffused light of the sun for at least several hours.
 * Telepathy: They have a telepathic link with the people they mimic, making them able to 'download" the memory and thoughts of this person as long as they are alive, which might explain why they don't intend on killing their victims.
 * Possession: Experienced eidola can take possession of other humans by slipping into their body while in their true slug-like form through their ears, nostrils, and throat. While possessed, their victim's eyes turn into a toxic gold and their voice turns "hollow and haltingly". They escape their possessed victim body when exorcised as a "dark wisp of smoke".

Enhanced Condition: Like most daemon, eidola possess enhanced physical abilities than normal humans, on-par with skilled athletes and soldiers but beneath that of those supernaturally enhanced. However, these abilities of theirs are only available to them while they are either in a human form or possessing a human.

Weaknesses

 * Retinal Flare: Eidola can be identified by a silver-gold retinal flare that shows when they are filmed or when light rays across their eyes.
 * Animals (Dog): Like most daemon, their presence can be identified by certain animals. In the case of eidolon daemon, dogs can identify them regardless of what forms they take, and the sight of them can turn even the most harmless dogs into deadly violent beasts.
 * Physical Harm: Like the majority of daemon and supernatural creatures, eidola can still be killed by mundane means if they have received a large amount of immense physical damage or received fatal and non-regenerable wounds on their human form. They can also be easily killed by mundane weapons and methods alike if they are in their true form, in which they have little to no way to defend themselves.
 * Hollow Ground: As daemon, eidola are incapable of entering hollow grounds, such as churches, synagogues, or mosques, without physically burning themselves with every second. If ingest dirt taken from these places, they will be poisoned, weakened, and their stomach will slowly melt away and their innards fall out.
 * Special Weapons: Weapons crafted from special materials that carry religious, mythological or supernatural symbolism or just simply be imbued with magical/mythical powers can be used to kill eidola. The most common of these weapons are silver-crafted ones, such as bullets or daggers, typically used by exorcists and hunters alike.
 * Heavenly Weapons: Like all daemons, eidola are extremely vulnerable toward weapons or artifacts bearing heavenly essence, not just light weapons or holy-empowered ones. A single slash from a dagger bearing "Heaven's blessing", i.e. blessed by a Catholic priest, is enough to immediately incinerate a High-class eidolon with much ease.

Trivia

 * They mainly live in the Belial territory in the Netherworld and in Acedia in the Underworld.
 * Some eidola might even work for pagan gods in exchange for human blood and flesh.
 * In the mortal world, they often make their lairs in dark and damp places like sewers.
 * They are native to Mediterranean regions, especially in the countries of Greece and Egypt.
 * Their skin can secrate a type of slime-like sweat which can transform into any clothing they want.
 * They can only breed with human women and the offspring is almost always another eidolon.
 * It is unknown if a female eidolon can breed with a human man, though it's implied to be possible.
 * Tessa Gray is a half-shadowhunter/half-eidolon hybrid rather than a full-on daimon despite having a powerful eidolon father.
 * It is later revealed that her father isn't an eidolon but a demon, the original Belial himself, making her a cambion.
 * An American eidolon named Charles Douglas had a warlock twin brother, suggesting that in cases such as twins or triplets, not all children will be born an eidolon and some might be warlocks.
 * Unlike other demonic creatures, they are incapable of noticing others of their own kind.
 * Despite popular belief, they are neither ghosts or undeads.
 * While they are known as "skin-walkers", they don't seem to have any similarity with the monster of the same name from Najavo mythology, other than the shared power of shapeshifting.