Forgotten One

The Forgotten One is a monster created by mortal hands, sealed in the Prison of Eternal Torture due to its uncontrollable power and to the fact it nearly ended the world. In order to fight it Leon Belmont had to first obtain the Unlock Jewel found in the Pagoda of the Misty Moon, then go down the stairs across from the warp room. He is the hardest boss in the game as he has three stages of battle (organs, arm, head) with 4800HP each. When destroyed, it yields the Black Orb.
Strangely enough, it is weak against Ice, making the Whip of Ice more effective against it than the Vampire Killer.
Before facing the Forgotten One, it's a good idea to have a full inventory of Uncurse Potions, Serum, Potions and High Potions. To reach this stage, take the south exit from the main portal room. At the bottom of the spiral staircase is a Save Room, a High Potion, and a door 'sealed by immense power'. Use the Unlock Jewel to enter the Prison of Eternal Torture. At the bottom of the stairwell, you will arrive at a boss room. Enter to fight the Forgotten One.

Boss Fight: Forgotten One

This is perhaps the most challenging of all bosses found in the castlevania universe. The forgotten one has 3 different stages, with unique attacks for each of the stages. At first you might think it easy to get through, but as you progress in the fight, you will find he is truly a handfull to deal with. The first stage of the fight, you will be dealing with his torso. Similarly to Beezlebub in Symphony of the Night, you will have to hit off his rotting organs. while you are trying to do this, there is a endless swarm of maggots who will be more than distracting to you. In Crazy Mode, these little guys will curse you if you come in contact. Even in easy mode, it is best to use something like Sonic Edge to remove them and get back to the monster at hand. His second stage consists of mostly fighting with his massive arms and hands. Forgotten One now has some rather nasty moves. One of these moves is his Arm Swipe. He will swing his arm across the entire platform, for some really large damage. This can be deflected with Perfect Block, so hopefully you have mastered that move. In Crazy Mode, his arm will shoot across the platform 3 times. If you have a hard time with Perfect Block, you might not survive this part of the battle. Other than swinging his arm across the platform, he also loves to slam his hand/fist down on the platform for some damage as well. This can be missed by jumping at the right time. If Leon is not on the platform when Forgotten hits, he will not be damaged. He will also shower the platform with poisonous blood. Once he does this, he will again slam his hand on the platform. If you get poisoned by the attack, simply try to wait until after he slams his hand down. This way you can (hopefully) get a item to cure you before he strikes the platforma again. This one isn't too bad if you have the attack pattern down. Forgotten's roar will bring a hail of boulders down on Leon. Avoid them as they fall. Now utilize the boulders protection when Forgotten breathes fire across the platform. Each boulder can be used as guard against the fire only twice. One the third time, it will explode and inflict some major damage if you are too close or behind it. With each new fire breath, seek out new bouldes to hide behind. If you get hit with the fire breath, you are almost certain to die. Stay safely hidden behind the boulders, and only attack when you truly can. May seem like an impossibility to defeat this mighty demon, but you can do it with time and patience. Once you ahve crushed Forgotten, you will gain the Black Orb. This is perhaps the best orb found in the game.