Daemon Soul: Dark Souls (Per)

Devil Soul: Dark Souls ( 悪魔・死界(デビル・ソウル・ ダーク・ソウルズ) , Debiru Sōru: Dāku Sōruzu lit. Devil: World of Death) is a Take Over spell that enables the user to transform into a unique 'demon'. It could be considered a subspecies of Devil Soul.

Description
Devil Soul: Dark Souls is a unique Take Over spell; as it does not properly transform the user into that of a demon and the like—but rather, it morphs their atomic structure and magical powers into that of the personification of death. When harnessing the power of Dark Souls, the user takes upon the form of that of a completely black humanoid with red eyes and a vicious looking face on its chest taking the place of its head, with razor-sharp claws and a tail. It should be noted that the 'personification of death' is not the Grim Reaper, otherwise known as The Boatsman; but rather, it is the power of 'granting death through every minor infraction'.

Dark Souls is special in that its creation came about through a million to one chance- the current user of the form, Shay Blacksquall, was fighting the Plaguespreader Etherious Blacker, he attempted to utilize Take Over on it in order to acquire a Devil Soul, but instead, it morphed into its own form, a Take Over Soul unique to Shay. It could be said, as such, Take Over: Dark Souls is one of the Black Arts, as the Etherious were created from said black magic and, as redundant as it is, Shay absorbed the plaguespreader. In addition to the usual physical boosts granted by assuming a Take Over form, Dark Souls grants the user the ability to perform Death Magic.

Powers and Abilities
Death Magic (死の魔法, Shi no Mahō) is a Black Art which enables the user to kill things with a deadly black miasma. Death Magic is one of the most noticeable Black Arts; and when one thinks of the term itself, it's almost immediately associated with the ability for a user to kill anything near them. This Death Magic is not to be confused with the Ankhseram Black Magic (矛盾の呪い (アンクセラム・ブラック・マジック), Ankuseramu Burakku Majikku lit. Contradictory Curse); which was bestowed upon select individuals by the god known as Ankhseram; this magic could not be learned, but instead, true to its name as a curse, it is a punishment for a person making light of the connections between life and death. In any case, Death Magic is considered to be a drastically weakened version of the Ankhseram Black Magic; in fact, in terms of basic function, the two are identical, but due to the absence of the influence of the god regarding its origin, the drawbacks –as well as the incredible power- of the magic are virtually absent; it could be referenced as a Subspecies Magic of the black magic indeed.

Death Magic works by utilizing their foul magic signature which is drawn from the fact that Black Arts users manipulate life, therefore their Magic, after being exposed to so much life-altering things, has taken on properties of 'death'; essentially causing the Death Magic to initiate the process of slow decay within living beings; essentially causing death in a limited scale. This power takes the physical form of a black miasma which radiates from the casting point upon the user's body in the form of a wave. This wave slowly causes the life of anything that it comes into contact with to deteriorate. In addition, with a high enough mastery of this magic, the user is capable of manipulating the shape of the wave released.
 * Murder World (殺界, Sakkai): Effectively, Dark Souls transforms the terrain around the user into a 'world of death'; as from the moment that the user initiates the spell, everything in the immediate vicinity becomes fifty times more dangerous and more likely to offhandedly kill the opponent; inducing the power of 'granting death through every minor infraction', such as being struck by a stray bullet, or falling from a large height by accident.. With Dark Souls, the user is capable of taking upon the form of numerous causes of death; via the principle that in this world, death, especially implausible ones, are unnatural. The most common form that the user is takes is the plague, which spreads across the ground, poisoning everything within its path; or they could be able to initiate the effect of the opponent being attacked by a stray blast; killing them in an undignified manner. All in all, thanks to the conceptual nature of Dark Souls, the user possesses a myriad of methods of attacking; giving them enormous versatility in any given situation.
 * Plague Master: The user can induce 'death by insects'; thus giving the user the ability to detect and control a wide variety of bugs and buglike creatures (including insects, spiders, earthworms, and crustaceans). Thanks to Dark Souls, the user is able to sense things through their bugs across a radius of three city blocks or more. As a result, they are able to track everything that is happening in their territory. Due to the powers of Dark Souls, the user can slowly flay their opponent alive with their bugs, have them burrow through the opponent's eyes, and even try to slip bugs inside the body to cause internal damage. The user can also create the possibility of anaphylactic shock (bee stings), comas (black widow spiders), tissue necrosis (brown recluse spiders) or death (all of the above).

Trivia

 * Shiva the East? More like Shiva the Deceased.