Death Devil

The Death Devil (の) is the strongest Devil who embodies the world's ultimate fear: death.

She is the eldest of the Four Horsemen, first mentioned by her younger sister Makima and later again by Yoshida.

As the Ultimate Terror (の) and Great King of Terror (の), she is prophecied to descend to Earth in July 1999 and bring about the apocalypse, which would end humanity and usher in an Age of Devils.

Appearance
The Death Devil's appearance is unknown, but she is female like her siblings.

Personality
Nothing is yet known about the Death Devil's personality.

However, per Nostradamus' prophecy, the Death Devil is to bring upon the apocalypse, end humanity, and usher in the Age of Devils (の). Seeing as her younger sister believes killing Death to be the only way to stop her, it seems probable that the Death Devil does not care for humanity and might, at the same time, expressly desire for Devils to dominate Earth.

Makima
Makima, the previous Control Devil, sought to kill all three of her Horseman sisters to better the world for humanity (or at least claimed as much).

Fami
The Famine Devil aims to prevent humanity's extinction as per the "Prophecy" by killing the Death Devil. Though Fami seems to believe that the events foretold, up to and including her elder sister's arrival as the Ultimate Terror, are inevitable (and has been shown to partly involve herself in their orchestration, like the Falling Devil's appearance), Fami ultimately plots to empower both her younger sister, the War Devil, and the Chainsaw Devil by inciting global fear of both so that ultimately, they may stand up to Death come July 1999.

If the Devil Death is aware of Fami's agenda to kill her is unknown.

Nayuta
Nayuta, the newest incarnation of the Control Devi, is well aware of Death's status as the strongest Devil. She noted that she and Fami, as her younger sisters, can tell that the Death Devil will indeed descend to Earth, while also deducing Fami's scheme to stop Death by empowering War and Chainsaw Man. However, Nayuta has yet to directly involve herself with Fami's plan to kill their strongest sibling, because she would rather go to school.

Yoru
Yoru as the War Devil, has not made her position regarding the Death Devil and the Prophecy known yet. Regardless of Yoru's own opinion, Fami and the Chainsaw Man Church plan to pit her and Chainsaw against Death.

Pochita
The Four Horsemen are said to have engaged in a brutal fight against Chainsaw Man in Hell, with the Death Devil's degree of involvement being unknown.

History
At some point in the past, the Four Horsemen sisters (alongside the Weapon Devils) were locked in a brutal, all-out fight against Pochita (the Chainsaw Devil) in Hell, at the height of which he vanished before their eyes. Unbeknownst to the Horsemen, Pochita had escaped to Earth in a completely changed, near-death form, hence why he could not be found despite searches conducted by the Control Devil.

While all her three younger sisters eventually found their way to the Human World where they parted ways, the Death Devil is still not present on Earth, her status and whereabouts being unknown. Regardless, her arrival is prophesied to happen in July of 1999.

Control Devil arc
Makima mentions the Death Devil alongside the other Horsemen while opening up to Kishibe about her plans to blot out the very concepts embodied by her sisters, by forcing Pochita to devour them.

Dating Denji arc
Hirofumi Yoshida is revealed to know of the Death Devil's existence. While they are held captive by the Eternity Devil inside an aquarium, Yoshida asks Denji (Pochita's host) if he reckons that Chainsaw Man would be willing to eat Death for humanity's sake.

Falling Devil arc
When Yoshida treats the Famine Devil to dinner, he queries about Nostradamus' prophecy, which holds that in July 1999, the "Great King of Terror" (Death) is to descend to Earth and cause humanity's demise. Fami confirms the veracity of this, noting that the first of the Devils who will shepherd the world to the "Ultimate Terror" has already arrived: the Falling Devil. The latter is revealed, not long after, to have been acting on Fami's orders. Fami explains to Nayuta, the new Control Devil, that she intended for Falling to feed the War Devil and her host, Asa, to the Justice Devil to starve and subjugate them for her plot. She reiterates that the King of Terror will soon appear, noting that she wants to prevent the resulting end of the human race by empowering her sister, War, to be able to win over said King.

Current arc
When apprehend by Public Safety, Barem Bridge mentions the prophecy again, before revealing that the Chainsaw Man Church has impelled 350,000 humans across the world to enter Contracts with the Fire Devil to transform into pseudo-Devils that resemble Chainsaw Man, all in preparation for the Death Devil's advent on Earth six months from now. Nayuta discerns that the church's and Fami's goal is to incite global chaos and thus raise humanity's collective fear of War and Pochita, to make both Devils strong enough to stand up to Death; the most powerful of all Devils and eldest of the Horsemen sisters.

Abilities
The Death Devil is held to be the strongest Devil in existence, her name most feared of all. While lesser "Primal Fears" are already transcendent beings that cannot be killed and whose mere presence can be felt across the world, the Death Devil's arrival is seen as tantamount to the apocalypse. Indeed, the prophecy of Nostradamus calls her "Ultimate Terror" and "Great King of Terror" — who is to usher in an Age of Devils and thus end the human race.

However, her sisters seem to believe that killing Death is within the realms of possibility: Makima desired to kill all her sisters by having Chainsaw Man eat them, and Fami plots to empower the War and Chainsaw Devils by rapidly reigniting humanity's collective fear of them (by means of inciting a global war) so that both can grow strong enough to rival Death.

The Death Devil can be assumed to hold all standard Devil abilities, like healing via blood consumption, forming Contracts, etc.

Devil Powers

 * Nigh-Absolute Immortality: Devils of Primal Fear status are considered unkillable due to their power, hence why they never undergo reincarnation. The Falling Devil affirmed that, at present, humanity has no means of attack that could kill a Primal Fear.

Trivia

 * The notion that the Death Devil embodies the ultimate fear maps onto real life: fear of/anxiety about death and mortality (also called thanatophobia) is one of the oldest and most innate fears, if not the oldest, is ubiquitous in the animal kingdom as a basic aversion, and underlies many other instinctual fears like that of pain, predation, disease, starvation, etc.
 * Predatory death anxiety, stemming from fear of harm, is the most primal death anxiety whose origins lie with unicellular organisms (e.g., protozoa) and their rudimentary, self-protective response mechanisms: their receptors evolved to react to stimuli to raise the likelihood of survival in the face of physical/chemical threats. Into humans, aversion to death has been wired through millions of years of evolution, and particularly the predatory kind of death anxiety may be evoked by whatever scenarios put one at risk or threaten one's survival, usually triggering fight-or-flight responses.
 * Scholars distinguish various forms of death anxiety, like predatory death anxiety (fear of being killed), existential death anxiety, personal fear of death or the dying process, anxiety towards the death of someone close, etc. Existential death anxiety is thought to be the most powerful form, founded in the basic awareness of human mortality that arose in humans some 150,000 years ago and sets them apart from animals. Since then (in a short span of evolutionary time) human psychology has fashioned a basic coping mechanism against this existential fear: denial.
 * The Death Devil being the last Horseman to appear mirrors how in Christian eschatology, Death is the final of the Four Horsemen of the Apocalypse, riding the Pale Horse.
 * While explaining Pochita's ability to erase concepts by eating the Devils incarnating them, Makima stated that there used to exist "4 outcomes other than death at the end of a living being's lifespan" (「がをえるとのにあった４つの」), until Chainsaw Man devoured the corresponding Devils. What these entailed in relation to Death and if their erasure has influenced her strength is unknown.