Black King is an alien kaiju that has appeared as an antagonist in several Ultraman installments, with his first appearance being in Return of Ultraman.
Return of Ultraman
Black King accompanied Alien Nackle to Earth, serving him as his bodyguard. After Seagorath and Bemstar were defeated, Nackle sent out Black King. MAT attempted to combat the monster, but their weapons were of no use against it.
Black King later went after Ultraman Jack, quickly gaining the advantage in the fight due to his impenetrable armor. Alien Nackle also showed up to assist, and the two succeeded in taking down and capturing Jack, taking him back to Planet Nackle to be executed.
Before the execution could be carried out, however, Jack was saved by Ultraman and Ultraseven. The three Ultras then went back to Earth to stop Nackle and Black King. Ultraman Jack emerged triumphant from the battle, decapitating Black King and destroying Nackle.
Black King appears as one of the monsters serving under Juda and having been modified as Remodeled Black King.
Black King appears in a flashback as one of the members of the Empera Army. For an unknown reason he is colored gold.
Mega Monster Battle: Ultra Galaxy Legend The Movie
Black King was borrowed by Alien Shaplay to stop Rei and the Ultra Brothers. In response, Rei summoned Gomora to fight Black King, and Gomora killed Black King with a Super Oscillatory Wave.
Black King was among the Kaiju transformed into Spark Dolls by Dark Lugiel. His Spark Doll was later found by Hikaru. Playing with it caused Hikaru to transform into Black King. Soon after, Black King encounters Thunder Darambia and fights him. After being beaten down, Hikaru swapped Black King's doll for the Ultraman Ginga doll he found, reverting Black King to his doll form.
Black King was later used again by Hikaru to fight Dark Galberos in a wrestling match in episode 7.
Black King likely returned to space after changing back to his original form when Ginga defeated Dark Lugiel.
Black King rampaged in area T-9. Xio tried to stop it but their weapons were of no use against the Beast. However, Ultraman X showed up to fight the kaiju. Alien Nackle Bandero soon appeared and assisted Black King fighting against the Ultra. However, Ultraman Zero appeared via dimensional travel and knocked off Black King's horn, prompting Black King and Nackle to retreat and take Rui with them.
Black King was later modified with cybernetics into Black King Drill Custom and was used by Nackle Bandero to prevent Ultraman Zero from saving Rui. The two fought Zero, and were eventually joined by Ultraman X. X succeeded in destroying Black King with a Xanadium Ray, while Zero defeated Nackle Bandero with his Zero Twin Shoot
Black King's Spark Doll was later absorbed by Greeza in the finale. However Black King, along with the other Spark Dolls, merged with X, giving him the power to defeat Greeza. Following Greeza's death, Black King and the other Spark Dolls and Cyber Cards thanked X.