Draugr

Draugr (復活竜 (ドローグル) Dorōguru, Old Norse for "Again-Walker" and Japanese for "Reanimated Dragon") is a form of failed reanimation. The term is specifically used to refer to Dragons who are revived through reanimation magic such as Preta Magic. Due to the inherently powerful magic nature of Dragons, they are incapable of being truly resurrected; a case of an untouchable soul. Due to this, any attempts to revive a Dragon will not call upon the soul, and only create an empty shell bent on destruction.

Draugr are the most feared form of Undead, rivaling, or perhaps surpassing, the fear that even Vampires instill in the human race, and the creation of a Draugr is expressly forbidden by the Magic Council due to this primal fear.

Overview
A fundamental rule of magic and alchemy is to avoid messing with the afterlife; all life is precious, and sacred, and should be treated as such. While some magics allow the mages to break the limitations of the living world and call back souls that have long since passed on, once in awhile, humanity receives a grim reminder that matters of life and death are not to be taken lightly. The very personification of death and destruction, the Draugr are a unique and powerful form of the Undead.

Draugr are the end result of any attempt to revive a deceased. Due to the inherently magical nature of a Dragon, the soul cannot be recalled from the afterlife, and a Jibakurei (地縛霊; lit. Earth-bound spirit) remains unaffected, despite séance magic such as the being able to communicate specifically with their souls. As such, when the revival of a Dragon is attempted, the magic backfires. Instead of a true resurrection, the end result is what is known as a Reinakitai (霊無き体, Soulless Body). The reanimated entity, as the name implies lacks a soul; it is a creature of pure destruction and instinct, and heeds the orders of no individual, acting entirely on its own whim.

Related to their corrupt revival, Draugr produce a powerful, incredibly lethal miasma that smells of death and decay. Anything within the range of this miasma will die within minutes; the precise time varies between individual, species, and proximity to the gas. Plants die when exposed to the miasma, turning black and shriveling, while animals who inhale it will die as the miasma passes through their system like a poison; mages seem to possess an inherent weakness to this poison, suggesting it works similarly to, however, it does not kill as quickly. This miasma has led to Draugr's gaining an additional moniker; Black Pestilence (運び死, lit. Hakobishi).

Biology
A Draugr, despite being a soulless shell, does have a body that mimics the biology of a true Dragon. Despite this, they do not need their vital organs to function so that they can be allowed to 'survive'; these organs serve only to fuel their immense magical energies and allow for them to perform Dragon Magic. A Draugr's eyes still function to allow for sight, but their pain receptors within the nervous system no longer function, allowing the creature to fight endlessly without feeling a sense of pain.

While a Draugr may take on the characteristics of the Magic that revived it — i.e. a Draugr revived through Preta Magic would be made of earth and ash — they also possess an immense healing factor, and are seemingly made of some kind of imitation flesh. When struck, the Draugr heals instantly, taking only a few seconds to recover entire halves of their body, and milliseconds to recover only limbs.

As is the case with true Dragons, a Draugr is barely affected by magic, only seemingly annoyed by even the strongest of spells; even the likes of Satan Soul only serve to annoy the beasts, and cause no actual harm — and any harm that would be caused is mitigated by their supreme healing factor. Even the likes of Etherion, the Council's strongest weapon, only incur the wrath of these terrifying beasts.

Abilities
Draugr possess the standard abilities of Dragons; flight, senses that far surpass humans, and the ability to perform Dragon Magic (竜魔法, Ryūmahō). Dragon Magic is the unique magical abilities possessed by all Dragons to control and manipulate elemental abilities, giving them unparalleled control and immunity over the element in question.

Additionally, the revived Dragon possesses whatever unique skills it may have had in life, such as ability to create powerful and deadly.

Weaknesses
Perhaps the one saving grace of fighting a Draugr is the standard weakness they share with true Dragons; weakness to Dragon Slayer Magic. As a magic specifically designed to kill Dragons, the Draugr, former Dragons who have lost their glory and been revived by the occult, fall prey to this magic as well, and it is by far the easiest method in which to harm a Draugr. Despite this, simply using Dragon Slayer magic cannot kill a Draugr; they can regenerate from anything that is not enough to destroy them completely, and as such, it generally would take the work of multiple Slayers combining their skills and effort to take down a Draugr.

Hinting at them being far more than a simple reanimated corpse, Draugr possess a shocking weakness to both Devil Slayer Magic and Demon Slayer Magic, showing the creatures possess a demonic trait within their bodies that makes them vulnerable to magic that slays creatures from the depths of Hades'. Devil Slayers are notably far more powerful than the average Demon Slayer, and it takes considerably less effort on their part to handle a Draugr. The rarity, however, of both these Slayers makes their status as ideal weaponry moot.

Restriction can hinder a Draugr as much as it can a living creature; for example, a Draugr will leave it in a suspended, frozen state until thawed by an outside force.

The last notable weakness possessed by the Draugr is a weakness to any form of purification. As a creature that personifies the negative traits of death and destruction, as well as despair, Light and Holy magics are a sufficiently powerful counter towards the Draugr, capable of deterring them, if at the very least not disintegrating them totally, and they are also incapable of entering holy areas that have a large amount of history behind them.

Trivia

 * The Draugr are based on the Necromancia of Dragonar Academy.
 * In Norse Myth, Draugr are a form of undead that possess numerous traits; one of many being the obvious stench of decay and another being found particularly around their own graves.
 * The term itself is also used in Skyrim, fittingly used for undead opponents.