The King of the Netherworld (also known as the King of the Underworld in the Japanese version) is a powerful and demonic, dark entity who is the consequential ruler of what is known as the Netherworld and a major antagonist in the Yu-Gi-Oh! 5D's series.
At first, he was only mentioned briefly by the Dark Signers' leader, Roman Goodwin in his rematch against Yusei Fudo. Roman described him as the "Ultimate God", the one said to be the mightiest of them all, and the Earthbound Immortals serve him. Also in the games this character has said to have been related to the Wicked Gods.
According to Roman, it would appear if the four control towers for the original Ener-D Reactor are not activated before sunset, as by then the door to the Netherworld will be opened. After Akiza fails to seal the Lizard Tower before sundown, the Condor geoglyph then appears in the sky towards the direction to where New Domino City is. After this, as the original Enerdy Reactor's light emanates from inside the B.A.D area crater, the King of the Netherworld emerges from it.
At first, it takes the form of a giant mass of dark slime, but then begins to mutate and acquires another distinctive shape. While the King of the Netherworld starts on its path toward the formed Nazca Line, Kuribon warns Luna that they must not let it reach the Condor geogylph. The Crimson Dragon also appears and transports the Signers, along with their companions, to the Stairway to the Dragon Star is, where Rex Goodwin himself is waiting.
As he reveals his true identity as the final Dark Signer, with the Condor mark glowing upon his back, he explains that he must complete a ritual for the King of the Netherworld to gain its full power.
Goodwin then proceeds to declare a Shadow Turbo Duel, which will be held within the Condor geoglyph, as the ritual to welcome the Ruler of the Netherworld, one which ultimately requires Signers to be sacrificed.
After Rex Goodwin, summons Earthbound Immortal Wiraqocha Rasca, the King spots it and dispatches an army of smaller condors to attack Yusei, Jack and Crow (although in "Signs of Doom, Part 3", King of the Netherworld managed to take out Jack on Goodwins order). However, they are fought off by Ancient Fairy Dragon and Black Rose Dragon, who have been able to be summoned by Luna and Akiza.
After Yusei finally defeats Goodwin, the Crimson Dragon arrives, using Yusei's Majestic Star Dragon to gain physical form in order to attack the King. The Netherworld's ruler attacks them with a massive beam, but Yusei and the Crimson Dragon pierce through and enter the King's body.
The moment Yusei is inside of it, he sees the defeated Dark Signers along with Goodwin, who has returned to his old self, as well as his brother Roman.
As the brothers thank Yusei, they informed him that the other Dark Signers will be revived. However, both brothers decide to remain. After they cross over into the light, Yusei and the Crimson Dragon pass right through the King, which disappears soon after as a golden ray of light emerges from its body, vanquishing it.