The boss of Boletarian Palace.
He is also known as the False King. The player faces him as a red-eyed demon created in the image of the King Allant. He wears a white tunic and wields a soul-form version of the black sword Soulbrandt. He can also cast the spell soulsucker which steals a soul level from the player.
King Allant the XII was once a magnanimous ruler who cared greatly for his subjects and fought diligently against the vile and the depraved. Unfortunately he became trapped in melancholy in his old age and searched tirelessly for a method to expand his might. Allant's search ended at the Nexus where the power of the Soul Arts was bestowed upon him. It is unclear who was his teacher, most likely the Maiden in Black (who has a deep connection to the Old One) or one of the Monumentals who perished. Allant's ability to use Soulsucker, the Maiden in Black's signature spell, makes her the more likely suspect. He returned to his Northern kingdom of Boletaria and with his knowledge of the Soul Arts brought unprecedented prosperity. It was at this point the King took the black blade Soulbrandt, a baneful weapon which grows more powerful the more demonic the soul who wields it becomes, from the Royal Mausoleum. Since that day the Soulbrandt has never left his side.
Elementary knowledge of the Soul Arts was not enough to quench his lust for power so Allant eventually returned to the Nexus and awoke the long forgotten evil, the Old One, from its eternal slumber. As a servant to the Old One, King Allant was granted unfathomable power which slowly mutated his corporeal form, eventually transforming his body into a pathetic, toad-like creature. Using his newfound power, he fashioned a Demon doppelganger to lead the Demon scourge and reap more souls to quench the bottomless thirst of the Old One.
Slowly the colorless Deep Fog began to spread throughout the land and wherever it touched, men began to lose their soul and with it their sanity. The mad attacked the sane and chaos reigned. King Allant believes that the Old One is a gift of mercy from God. So much pain, evil, and misery fills the world and the solution is to fight that poison with poison. Once the colorless Deep Fog has engulfed all the land and absorbed every last soul, human existence will end and with it our suffering.
The False King's Demon Soul is used to fuse together the Soulbrandt and Demonbrandt into a single weapon, the fabled Northern Regalia, the original emblem of royalty (like a crown or scepter) of the Northern Kingdom. The weapon is most potent when the wielder is either at their most pure or their most depraved and evil. The Northern Regalia is as ancient as the Old One and, like the Old One, it was left in the world for malicious purposes.
Ostrava of Boletaria, his son, was searching for him and after being severely wounded by the demon impersonating his father. Ostrava gave the Mausoleum Key to the Slayer of Demons so that the hero could free Old king Doran and get the legendary sword, Demonbrandt, to kill Allant and save Boletaria.
Allant, driven by a lust for power, became a demon after he made a pact with The Old One. This boss is known as the 'False King' because it is not the REAL Allant. After Allant allied with the Old One, he was given immense power. Using this power, he fashioned a demon made in his own image, made to look like his old self at the height of his power. This demon would lead the demon armies against Boletaria.
Below the Nexus, you can find the physical form of King Allant, which has been twisted and contorted into a toad-like creature by the power of the Old One. Much like the Old Monk, King Allant was not a suitable vessel for Demon power. The process horribly mutated his body into a weak, toad-like creature which can only crawl along the ground and barely swing the sword Soulbrandt. The Demon which was sent back to rule Boletaria, the False King, was an imitation, a doppelganger similar to the Fool's Idol.
What the player fights inside the Old One is the body form of Allant. The fight with Allant is incredibly easy, which is the point, the final boss is a cautionary tale to those who desire to go down the dark path and serve the Old One in exchange for power. In his lust for power it has cost Allant everything and left his body broken. Even in his enfeebled state he still clings to the Soulbrandt. Does the player take the hint and go for the light ending or do they arrogantly believe that they are special, different than Allant, and take in all that power through the dark ending knowing where it will likely end?
The mercy killing of Allant also makes a good thematic comparison to the Maiden Astraea boss fight in the Valley of Defilement (which many leave for last). In that fight the player murders a human who is just trying to protect his love, and she commits suicide as a result, a somewhat dubious act. Similarly, even if Allant was a bad man, there's just something inherently vicious about killing the broken shell of a man who can't even fight back.
It would appear it was a combination of his depression as he got older and his lust for power that was his downfall. He projects his own personal suffering and melancholy as the world's pain and suffering and becomes a servant of the Old One in order to end all existence, and with it, all suffering.