In ancient times when the world was just created by the gods, the Great Serpentine Devil introcuded evil in the hearts of man, and the concept of death was revealed to them.

Several people witnessed a red giant flying beast made of darkness lay claim to many lives wherever it flew, and a wise blue woman set out to seek the mighty beast's lair.

In her travels she came across three lake spirits, representing Will, Knowledge and Emotion.
The lake trio decided to help the blue woman in her quest and together, forged the mighty Trinity Blade, the only weapon capable of harming the vile red beast in its current form.

With the Trinity Blade in tow, she finally reached the red beast's lair.
Confronting the mighty red beast, he mocked the blue woman, explaining that no matter how many times he will be beheaded, he will simply reincarnate and continue claiming lives.

The wise blue woman did not behead the red beast. She impaled his skull instead, stating that no matter how many times the red beast will reincarnate, the blade will act as a seal to his power and only one with a pure heart can remove the blade again.

Enraged at his defeat, the red beast split into two. The half containing its raw power ascended to a higher plane of existence and became Desma.
The other half contraining its malice and physical form alongside the blade effectively died and fell to the Infernal Realm, where it would later reincarnate into Mephistopheles' Infernal body, after having discarded his Celestial body after falling from the Celestial Realm.

Desma became the caretaker of the Tree of Life, an enormous tree in a higher plane of existence that continuously provides the universes with life.
In order to keep the Tree of Life alive, Desma has to claim lives indiscrimately. He does so based on instinct rather than actual will.
The experiences of each life is absorbed into the Tree, sustaining it, and also effectively renews the life, which is then sent to be reincarnated in a different world, universe, dimension, or existential plane.

In modern times, the Great Serpentine Devil was slain, but is trying to resurrect himself by leeching life force directly from the Tree of Life.
Desma took notice that the Tree was in danger because of this, and instinctively defaulted to having the need to claim many, many more lives to correct this loss of life force. An entire universe worth of lives would be needed to save the Tree, even though it's merely prolonging the inevitable.

It was around this time that Mephistopheles, being incarnated into the beast's body, conquered planet Primale to live out his ideals, and, being the Essence of Despair, rapidly sunk into despair himself.
He eventually deemed the world to be unsalvageable and decided that the complete elimination of all life in the universe is the only solution.

He became hollow. Neither dead nor alive, his heart was purely empty. It was this purity of heart that allowed the sword in his skull to be removed.
Desma needing the life force of an entire universe, Mephistopheles desiring the end of all life in the universe, possessing the red beast's lost half as well as no longer being cursed by the Trinity Blade, formed the perfect storm.
Desma descended and incarnated into Mephistopheles. With the beings of Death and Despair into one, Mephistopheles could finally achieve his goal. Even if his despair completely ate him away and is now more acting out on an instinct rather than an actual will, Mephistopheles is still in full control of the fusion.

With Mephistopheles' natural influence over people's despair now amplified with the power of death itself, the universe begun to commit mass suicide.
However, one Primalian girl managed to withstand the intense despair with her hope and confronted Mephistopheles.

To safeguard his plan, Mephistopheles opened a portal to the Realm of the Dead and fled to it.
While the Realm of the Dead would absolutely evaporate the soul of any living being who enters to the point that reincarnation is impossible, the young girl didn't hesitate to follow Mephistopheles.

As the only one alive of her universe, the young girl's powerful hope managed to gather the power of every single deceased soul to defeat Mephistopheles. However, one cannot kill death...
So she then travels into Mephistopheles' psych and battles one final opponent that forms the link between Mephistopheles and Desma and severs it.

The death process is then reversed and all life is returned to the universe. Desma becomes separated from Mephistopheles and returns to the existential plane he came from.
Now with nothing to sustain himself with, Mephistopheles is nothing more than a brittle husk and explodes.
The force of this explosion knocks the young Primalian girl into the portal back to her own world.

The portal to the Realm of the Dead then closes.
With Mephistopheles' body, that of the flying red beast in the past, now sealed inside the Realm of the Dead, it ensures that any access to the body is locked off, preventing such a tragedy from ever happening again.

Being a Formless, Desma lacks a true physical and visual form, as well as a rue name and identity because higher existential planes have mechanics different from that of ours. As such, Desma's true nature is unfathomable, and is often regarded as an invisible force of nature.
Many have tried to visualize Desma and attempted to portray him in art, which resulted in him gaining many powerful forms. Those who expect Desma to take on a certain form will see him in that form, even if it's a subconscious idea if his appearance. However, due to the nature of the existential planes, he will not gain an appearance outside of his own plane. Anyone who bear witness to any of his visual appearances have already died.

Because the residents of the Mortal Realm have the unique power of Perception, they have the ability to anchor Formless entities to a visual form and an identity, capable of greatly reducing a Formless' power through power of Perception.
Desma gained his main visual form and name after being Perceived by the young Primalian girl in his own existential tier.

Desma continues to be the caretaker of the Tree of Life to this very day, and will do so until no more life remains in the entirety of existence.

His robe is said to be comprised of evil life that committed acts so foul, they have been denied any further reincarnation, so Desma guards these lives from escaping, preventing them from reincarnating.