Archfiend Mode

Archfiend Mode (, Āchifīendo Mōdo lit. Hades Ruler Awakening Devil Power): Archfiend Mode is a peculiar Devil Slayer Magic ability that can be seen as an advanced version of Devil Force that calls upon the entirety of the demonic power that is stored within the grimoire that the Devil Slayer learnt their particular form of Devil Slayer Magic from, taking the power of the creator of the grimoire and the demon whom sealed its power within the book for their own and stacking it upon their own strength, thus boosting the Devil Slayer up to the level of a demon king and even beyond, granting them the ability to wield the power of the book's creator and the demon within in conjunction with their own magic.

Description
Archfiend Mode is perhaps the strongest transformation that a Devil Slayer is capable of undergoing – referred to as the end-be-all upgrade to their powers, even above the likes of Devil Force, it serves as the ultimate representation of the entwined destinies of the Devil Slayer, the one whom wrote the grimoire, and the demonic being whom had their dark energies installed within the book. The souls of the creator and the demon being inscribed upon the soul of the Devil Slayer after their training is complete, Archfiend Mode has the two entities have their existences combine with the soul of their sort-of-protégé's soul in a manner not dissimilar to the likes of the Dragon Soul Seal as displayed by Dragon Slayer Magic momentarily, before the demonic energies installed within their Magic Origin – both in terms of Curse and – as well as the supernatural power output and physical parameters of these two are extracted from these souls and shoveled into the Devil Slayer's Magic Origin and overall body. Serving as one final boost in power that transcends all others, this grants the wielder of Devil Slayer Magic the remainder of both the bookwriter and the demon's magical and physical power by modifying their biology and Magic Origin; the visual representation of which is a demonic tattoo that is branded upon their skin in a place that depends from user to user – the tattoo is the symbol of the Devil Slayer inheriting the full power of the one whom penned their grimoire and the demonic being with their powers stored inside the arcane book, fully completing the installation process of Devil Slayer Magic. one final memento left from parent to child. Alternatively, the user can get this ability as a parting gift from the writer of the book and/or the demon without the seal, as long as the originators of their power are deceased. This tattoo, referred to as the "Devil King Brand (魔王烙印, Maō Rakuin), is known to come in a variety of shapes and sizes, but oftentimes resembles their Magic Seal albeit with noticeable demonic additions to its design. As the Devil King Brand is the source of the vast amount of demonic power bestowed upon the Devil Slayer by their progenitors, it serves as a catalyst of sorts for them to unleash their full power. As such, through mental commands, the wielder of Devil Slayer Magic is able to draw out the powers of their progenitors and merge these powers with their own; this is known as Archfiend Mode.

In any case, when invoking Archfiend Mode, the wielder of Devil Slayer Magic focuses the voluminous quantities of arcane energy dwelling within their Magic Origin intently, but instead of inducing the usual activation sequence of their magic, they simply focus, delving deeply into their mind as to subconsciously look at the contents of their Magic Origin and their particular form of Devil Slayer Magic in particular. Focusing their mind while doing so, the Devil Slayer synchronizes their Magic Origin with the Devil King Brand through mental commands, causing a powerful resonance – the demonic tattoo begins to shine brilliantly in the texture of the user's element, causing the powers of the one whom penned the grimoire that allowed the Devil Slayer to obtain their powers and the demon whose energies are inscribed within said grimoire to influence the Devil Slayer's body and magical powers, thus resulting in a dramatic change. Physically and outwardly, the wielder of Devil Slayer Mode undergoes some subtle changes – for one, the sclera of their eyes invert in texture, transforming from white to black, and all the while, demonic energies begin to flow outwards from the tattoo. The user's Magic Origin is modified in order to take the addition of the demonic power of their progenitors into consideration, becoming larger and amplifying the quantity and effects of the magical power stored within. Additionally, the user's body strength is drastically augmented, with the tattoo altering it in order to take upon the properties of the demon at full power as well as the physical might of the writer of the grimoire, both sources of physical might stacking upon the Devil Slayer's own power as their speed and power drastically increase to levels that allow them to fight toe-to-toe with incredulously powerful beings. The sheer volume of the magical power granted to the wielder of Devil Slayer Magic by the invocation of this power-up, even without releasing it outwards in the form of their Magical Aura takes the form of a flame-like aura with the textures of their own element that wraps around their body – when the Devil Slayer decides to unleash their Magical Aura, this blazing aura becomes even greater, washing over the area and subjecting anything and anyone in the immediate vicinity to the effects of the caster's magical power – this aura seems to constantly discharge aftereffects of their element, such as a wielder of Ice Devil Slayer Magic causing the temperature to dramatically drop or the user of a pyrokinetic-type Devil Slayer Magic setting everything near them alight.

When the power granted by Archfiend Mode is exerted to its maximum, in a radius expanding meters, the surroundings itself becomes a storm of elemental energy which varies from user to user, and the user becomes cloaked in their element, which takes upon the manifestation of their demon, causing a frightening visage. Not only this, due to accessing all of the powers of the demon whose power was inscribed within the grimoire that they obtained their magic from and the writer, the wielder of Devil Slayer Magic has a vast malleable mass of demonic energy shroud their body which they are capable of molding into a variety of forms such as weapons and defenses, but oftentimes, the mass of energy is shaped into armour that fits around their body. Thanks to the overwhelming power granted to the Devil Slayer by Archfiend Mode, the sheer strength of their magical power is capable of shrugging off various forms of such as Black Arts and Curses, including the likes of, which is known to be a one-hit-kill on anything else. Not only this, when Archfiend Mode is in effect, the Devil Slayer is able to achieve a form of unlimited flight, using aspects of their element in order to propel themselves throughout the atmosphere. When in this form, the user has access to enhanced versions of their normal Devil Slayer Magic spells, referred to as "_ Demon King's _" (_魔王の, _Maō no_). However, in order to utilize Archfiend Mode to the utmost of its ability, it can take an extremely long time to learn the ins and outs of the power, with the absolute maximum taking ten months of dedicated training to properly master. Another weakness of Archfiend Mode is that the magical power bestowed upon the user by their progenitors is extremely finite, mostly due to the progenitors only giving them this power in their last moments in existence; and once it runs out, the user can never utilize its power again unless they utilize their own magical energy as a catalyst for its activation, in which case the Archfiend Mode will be noticeably less powerful.

Trivia

 * Archfiend Mode can be considered the Devil Slayer equivalent to Dragon Slayer Magic's Primal Dragon Mode.