Eidolon Devil Slayer Magic

Eidolon Demon Slayer Magic (化身滅悪魔法, Keshin Metsuaku Maho; lit. "Avatar of a Murderous Demon") is a very old and powerful Lost Magic, with records indicating that it has existed for thousands of years. In a time plagued by endless death and chaos humans communed with spirits for guidance and power, and those prayers were answered in the form of Eidolon Demon Slayers. Since then they have appeared scattered across time, rising only in times of great upheaval and war.

Description
Eidolon Demon Slayer Magic is a branch of Lost Magic which allows its users to transform the physiology of their body into those of a demon. Eidolon however is a unique type, since it does not utilize a specific elemental typically found in Demon Slayer Magic, instead the caster acquires the powers of a demon through what is known as the Covenant, an unholy contract binding the soul of the caster to the Demon. By invoking an incantation which serves as a summoning he can allow a portion of the Demons power to manifest through him, making this style of Demon Slayer Magic a Holder Type. The caster acts as a vessel for the Demons power and takes on many of its physical and psychological characteristics. Unlike other Slayer Magic, where the style is generally taught by the creature in question to the caster, Demon Slayer Magic instead uses the caster as an avatar or a vessel for the Demons power. Furthermore with every use of this magic, the caster's physiology begins to change, as the physical characters of their demon begin to manifest along with augmented physical abilities, and magic in their base human form. Furthermore the type of magical enhancements a Demon Slayer obtains depends wholly on the Demon they have made a contract with. Therefore Demons with a strong affinity for a specific element will bestow that same affinity and associated abilities to the caster. Ultimately the type of enhancements and abilities bestowed upon the caster are wholly dependent on what powers the Demon in question has.

The Covenant
Magic is the manifestation of thought and emotion connecting with spiritual flow of nature. Thus when a prayer is made, when there is suffering, anger and negative emotion, these gather into powerful forces which serve as a beacon connecting our world to those that exist beyond. Demons are beings not of this plane. They were neither born in the human world nor created through any form of magic existing in Edolus. They live beyond the gate separating these realms. They are creatures of otherworldly thought and body, and it is through this connection they are drawn to our world. Calling a Demon is notoriously difficult. They do not subscribe to our perceptions of morality and reality, so methods to contact them vary depending on the Demon in question. What is known, is that it requires a very powerful feeling of emotion to gain their attention. Magicians have gone years praying and offering sacrifices to entice these outsiders. Often they are drawn to scenes of immense negative energy or death. Once contact has been made, the Demon will appear before them in mind, dream, or some other form as they lack a body. The Covenant (盟約, Meiyaku) is a pledge, a binding oath between the magician and the demon they have contacted. It serves as the bridge that connects the Demon to this earthly realm and how the magician can draw upon its power. Sometimes there are conditions, the contract itself may outline instances in which its power may or may not be used, but ultimately whatever the oath is, the Demon gains a foothold into our dimension.

Slayer Magic
As its name implies Eidolon Demon Slayer Magic is a type of Slayer Magic, the powers that are at the command of any Demon Slayer are unique, born from the corruption and foul influence of their patron Demon. The very magic that empowers them is inherently evil at its core. It is not a magic of life, nor is it one of death, it is simply put a magic of unadulterated evil, and whatever the reason, whatever their current goal or aspiration use of this magic will corrupt its wielder and inevitably turn them into an instrument for powers beyond mortal comprehension. Their magic is potent, for the energies that compose it are of an unnatural kind, a sort that lies beyond the viel of the earthly realms, but it is this source that makes it so unique. Demon Slayer Magic, regardless of form, will be able to in varying capacities to harm those of demonic origin, including other Demon Slayer's. There are few monsters or beings that exist that can claim outright immunity to the powers of a Demon Slayer, though they may possess varying degree's of resistance, such as Dragons and other powerful ancient entities.

Invocation
As a Holder-type magic, the caster must first speak an incantation, which is specific to each demon as well as the covenant made. Once spoken this invocation acts as a ritual summoning, which serves as the gateway for the Demons powers where they are then bestowed upon the caster. Like typical Demon Slayer Magic, an invocation, is used very much like a spell, only in this case the caster is invoking the Demon to use its magical abilities using the caster as a medium.

An invocation is generally long-winded and poetic, but can be broken up as desired by the caster to partially summon the powers of his or her demon in to the material world; this method is generally preferred due to the inherent dangers of using this form of magic. Demons are entities that possess tremendous amounts of magical energy and summoning even a modicum of its power is draining to the caster on the physical level. Therefore most Eidolon Demon Slayers tend to be quite proficient in various forms of martial arts, and are highly durable mages.

Eidolon Demon Slayer Magic is by far the quickest means towards obtaining an instantaneous boost in one's power, however their power is not their own. Should they anger the Demon, it may withhold its power at its discretion, or even punish its caster. Furthermore extended use of Eidolon Demon Slayer Magic, slowly drains the life force of the caster, though they grow progressively more powerful in their base human form. There is also the danger that any given caster can be driven mad by the infernal powers they command, ravaging the landscape as tyrants, some even at the beset of their demon.

Spells
Demon's Baptism (悪魔洗礼式, Akuma Senreishiki): Is the invocation used by the caster to call upon the powers of their patron Demon. Each invocation is unique, and its effects are varied depending on the skill of the caster and the powers granted by the Demon itself. Regardless, the transformation is brutal and the form almost always grotesque to behold, but the power is immense. By virtue of the covenant, the Demon Slayer is able to draw on a small portion of their patron Demon's abilities. Depending on what the Demon was, the powers they are granted will reflect that. The most common of Demons known are those that can control the elements. However unlike other forms of Slayer magic, they do not consume the element so as much as they absorb the energies that give it form. They can replenish their energy by making contact with the element in question and then break it down and absorb the ambient magical energies. This process is referred to as The Devil's Feast (悪魔の供応, Akuma no Kyouou). Other Demons of strange powers exist that govern emotions or other concepts such as wrath or lust. These Demon Slayers draw their power from the intensity of these feelings around them, and more often than not will indulge themselves in propagating events to stir these emotions into a frenzy. The Demons that grant these magicians their favor often possess powers that are similar to those of other forms of magic; teleportation, swordsmanship, illusions, dreams, et cetera. When a Demon Slayer makes use of these magical disciplines they are empowered by their Demons magic, corrupted and tainted by its influence. These powers are order of magnitudes stronger but far less controllable, lacking the finesse of a skilled master but possessing far more than their might could ever bring. A Demon Slayer whose patron was associated with Dreams could put an entire populaces to sleep, and wreck havoc on their minds through nightmares, they could weave powerful illusions spanning an entire city. Because of the manner in which they draw their power, they grow ever eager to manipulate events into obtaining it, spiraling downward into madness and evil.

Drawbacks
As with any method to obtain power quickly, Eidolon Demon Slayers suffer from weaknesses and drawbacks. First and foremost is that by entering in the covenant, they have condemned their soul, it is no longer theirs, it belongs to their patron Demon in return for their power. Every time they use its power it gains a little bit more influence over their actions making them more addicted to the thrill of killing. They are avatars, embodiment's of their power and the goals of these strange entities are wholly alien to our own. As they delve deeper into the power granted, they begin to hear the voice of their Demon. At first it is like a soft whisper, indistinguishable, but pleasant. Then as they grow in strength the voice becomes clearer. It will converse with them, tempt them with even greater power by offering more of themselves to it. By offering sacrifices, to feed its power. Soon the Demon will give commands, and from then, they may black out as the Demon is able to take control entirely for limited spans of time. Also in order to maintains a healthy body and the abilities of the demon through the Covenant the user must consume the blood of other demons. Demon Slayers of this kind suffer from other weaknesses as well. Due to the nature of how their powers function, opposing elements such as Light are even more dangerous as they seem to act as a counter-balance to the magic of a demon slayer, so both magic's would cancel each other out Though they are unaffected by the normal magical cancelling/nullification effects as with their counterparts the Devil Slayers, their powers are more reactive to the presence of intense positive emotions. Their powers can actually be weakened by very strong feelings of compassion, and love. However the converse is also true, this means that they can directly gain strength from suffering, pain, and agony, and hatred which in a world filled with rival factions is easily abundant.