Forgotten One

The Forgotten One (忘れ去られし者 Wasuresarareshi Mono) is a monster created by mortal hands, sealed in the Prison of Eternal Torture due to his uncontrollable power and to the fact he nearly ended the world. In order to fight him, Leon has to first obtain the Unlock Jewel found in the Pagoda of the Misty Moon, then go down the large staircase located behind a mysterious door at the entrance of Walter's castle.

The Forgotten One is arguably the hardest boss in the game, as he has three stages of battle (organs, arm and head) with 4,800 HP each. When destroyed, he yields the Black Orb.

Strangely enough, he is weak against Ice, making the Whip of Ice even more effective against him than the Vampire Killer itself.

Background
The Forgotten One is portrayed as a gigantic being completely stripped of skin, with all his insides, flesh and bone, exposed. His head fashions two ox-like horns and two tusks, giving him the appearance of a demon; however, due to the fact that he is a "man-made monster", as the description says, he can't be considered to be one.

He is bound by gigantic shackles, hooks and chains on many parts of his body, retaining him suspended above the ground. An ancient mural found in the Pagoda of the Misty Moon's west tower implies that this monster was subdued and put away since immemorial times, with generations and cultures coming and going in the surface world, while the creature remained all that time imprisoned underground, suffering his fate (hence the name of the area where he's found at).

Due to the level of putrescence his body has attained all these years, gigantic maggots have formed inside of him, eternally feasting on his flesh (this feature had also been seen before in enemies like Legion and Beelzebub). His blood has been tainted by the many centuries of decay, and, as it seems pain is something he's very well accustomed to, it appears he has no problem in inflicting wounds on himself and use it as a means of attack.

The rage he has accumulated all these years allow him to unleash a powerful roar capable of shattering the very earth itself, causing rocks and debris to fall from above. His ultimate attack is an almighty and devastating energy beam that surges from his mouth, capable of destroying anything in its path.

Attacks
The Forgotten One's first stage is his lower torso and organs. Leon has to attack his organs to tear them off one by one.

Form 1

 * An endless supply of maggots will drop off of the Forgotten One's body, landing on the platform where Leon stands. If he is close to them, they will arc back before lunging at him. These maggots are capable of inflicting Curse status.
 * A large gush of poisonous blood will drip from either the left, right, or middle parts of his organs.

Form 2

 * He sweeps his hand across the platform, dealing heavy damage. On Hard Mode, he will sweep his arm three times.
 * He will hover his hand above the platform and slam it down on it. Alternatively, he will smash his fist down.
 * He clenches his fist wih great force, provoking a wound on himself that drips poisonous blood; he then continues to spray it all across the platform. After he finishes releasing the blood, he will slam his hand down again.

Form 3

 * He unleashes a powerful roar that shakes the room, causing rocks to crash down from above. Some bigger rocks will remain on the platform while the smaller ones will disappear.
 * He sweeps a devastating energy beam across the stage. The rocks are the only shelter from this attack. Being hit by this attack generally results in instant death.

Trivia

 * The Forgotten One has the second highest HP amount of any enemy in the Castlevania series, with a total of 14,400 HP. The only one that exceeds it is Legion/Nuculais from Curse of Darkness, with a total of 15,775 HP.
 * The Unlock Jewel (the key to the Prison of Eternal Torture) is hidden away in its very own tower located on the west wing of the Pagoda of the Misty Moon's second floor, guarded by Walter's most powerful monsters and near Walter's own throne room. Added to that, the room where the Unlock Jewel is held in, is a pitch black, bottomless room, with an ancient mural on a wall showcasing the Forgotten One as a warning to everyone who dares face him.
 * He has only one arm and, according to this mural, it was torn off since the times when he was imprisoned.
 * The Forgotten One is considered one of the most disturbing bosses in the Castlevania series.
 * The Forgotten One is similar to Beelzebub in that they are both giant enemies retained with chains and shackles above the ground, and both must be butchered limb-by-limb and organ-by-organ, in what ends up being a very gory battle.
 * The Forgotten One is Koji Igarashi's most memorable boss, in part because of how shocked the fans were, as well as the fact that his appearance bumped Lament of Innocence's rating to mature. His favorite boss gameplay-wise is Abaddon.