Victus Mortuus

"'I shall grind beneath my heel... all that exists.'"
Described by Princess Celestia as the 'ungodly corruption', Victus Mortuus is a demonic entity of incredible power, who was thought to have been defeated long ago by the combined efforts of Celestia and Luna's mother and Discord. He is considered the most brutal and sadistic adversary ever faced in Equestria, still capable of making the Princesses and Discord tremble at the mention of his name. His name is latin for 'living dead'.
Much as Detritus served Princess Celestia as the spiritual guide of lost souls and wandering spirits for the afterlife, so did Mortuus many centuries ago for the Equestrian Queen. Mortuus aided the lost spirits find peace in the afterlife, as well as help the old and sick find acceptance into death, effectively serving as Equestria's first grim reaper.
This peaceful exchange between Equestria and the Hell beneath it would not last forever. Each soul he helped ferry into the afterlife granted him a burst of power, and soon Mortuus would feel an insatiable appetite for more, causing disasters such as volcanic eruptions and earthquakes just to increase the death toll and thus fuel more spirits into his hellish home. The Queen finally would learn of Mortuus' growing hunger and soon attempted to have him evicted from his position and silenced into the sun, never to return to Equestria. Peace would return to the land for a few years after.
It is uncertain what became of him during his banishment. He had presumably lost all morality and emotion, letting his anger and lust for revenge break his body and soul into shards. He was completely berserk, and soon broke himself into the very core of the sun, letting his last bit of morality rip apart and transforming him into his grotesque and demonic visage. He broke himself free to return to Equestria, brutally attacking the Queen herself with burning strength before crushing her skull in his grip, usurping the throne from her.
Equestria became a broken country, continents shattered and scattered, oceans burning and devoid of life, soon the land became the same hell he once called home. He was soon challenged by the god of chaos for control, and was defeated thanks to his anger growing out of control against the calm and trickster nature of Discord. Mortuus was shackled back in the deep center of the sun yet again, Discord returning to rebuilding Equestria before losing control himself and usurped by Celestia and Luna. He would be locked forever in the burning sun by artifacts of power handed to six of their bravest guardians, later dubbed the Elements of Harmony.
Mortuus draws his power from the very soul and heart of the Hell he came from, fusing the searing heat with his pure brute strength for his purposes. He believes in the power of corruption, capable of transforming mortals into lesser demons with no possible method to return to normal aside death. He speaks with a deep voice with deeper echoes following, speaking in an old accent similar to the Queen and by extention, Princess Luna.
His brutality appears to be his defining trait, as he wasted no time in flat out bludgeoning the Queen of Equestria a good bit before crushing her head in with his bare hands. He is undoubtedly physically strong, and with his power of corruption, he is a near invincible enemy. There is no limit to his corrupting power, as even the purest of hearts will become the most bloodthirsty of murderers at his whim. The horror comes with the knowledge that there is no cure for it at all, and that the victim is doomed from the start. His mere presence is felt with a searing heat.