Octomus

The Supreme Master of the Underworld (AKA Octomus) is the main antagonist of Mystic Force and the true leader of the Forces of Darkness and his minions revere him so much that he has only ever been referred to by his title. He contacts his minions via the white pool at the center of their lair, which turns red to signal his presence. His true form was revealed to be an immense Cthulhu-like demon with a skull-like face, many tentacles and dragon heads on the end of each tentacle, but usually only his right eye can be seen. His other eye is embedded in Koragg's shield, and one of his fangs was used to make Morticon's sword (which was taken by Koragg when Morticon was destroyed and forged into a new Knight Saber) and quite possibly some of his claws were given to Necrolai, her "Claws of the Master".

He was sealed in his Pit by the efforts of Leanbow at the end of the Great War, when the Forces of Darkness attempted to invade the Surface Realm, but he was able to transform his enemy into his most loyal servant Koragg the Knight Wolf beforehand. He was also responsible for dragging Catastros down to the Underworld. With the Forces of Darkness active once more, their sole motivation is to free their Master.

The Master has often had to intervene to stop his minions squabbling - in one case, Koragg was placed on trial before him, accused of being a failure to the villains' cause, and he made his verdict in Koragg's favor. He also created Imperious out of the mummified remains of Calindor.

To gain the Master's favor after his attempts at usurping him had failed, Imperious proposed the idea of feeding the Ranger's Legend Warrior powers to him, giving him the power to enter the surface world. But when Udonna's interference almost got her killed by the Master himself, Koragg stopped him at the last second and was turned into Leanbow once more. The Master was able to partially emerge onto the surface before Leanbow pulled off the ultimate sacrifice, seemingly destroying himself and the Master. However, the Ten Terrors were confident that by following the Rules of Darkness, they could bring about Master' resurrection.

Nevertheless, in "Hard Heads," a monkey wrench was thrown into the Terrors' works. Sculpin learned that Leanbow, still stuck in the body of Koragg, seems to be somehow preventing the Master's resurrection from beyond the grave. Later, Sculpin located Leanbow in the Underworld, and extracted the Master's spirit, giving it to Gekkor for safe keeping. In "The Return," the Master ultimately chose Matoombo as his vessel/egg and was restored to having a physical body by hatching & tearing his way out of Matoombo's body and starts melting & molding it into his own evil image of a skeletal fallen angel-like beast with a skull-like face, a black mask that concealed his hideous brain and numerous tentacles. He also had two additional eyes on his skull-face to replace his original right eye.

In his final bid for power in "Mystic Fate," the Master first possessed Nick Russell directly and transformed him into a new Koragg - one more sadistic and vicious, who laid waste to parts of the forest and Rootcore before he was saved. While the Rangers were distracted by this, the Master personally went to the Mystic Realm and seemingly destroyed the Mystic Mother, source of all good magic, and when Leanbow & Daggeron arrived to face him he devoured their Knight powers and killed them. Following this, he personally went to Briarwood and attacked the Rangers directly, hurling them into a vision of the future where he had succeeded in destroying the Earth before devastating the Manticore Megazord, devouring part of their magic, and sending the Rangers fleeing.

His victory seemed certain - Nick was able to wound him and have the Rangers sent back to the real world, but at the cost of their magic. However, the Master was undone when it was revealed Itassis & Necrolai had defected from him (causing the resurrection of Daggeron & Leanbow), the Mystic Mother was still alive and both the people of Briarwood and the forest had gathered against him. Regaining their magic from people's belief in them, the Mystic Force Power Rangers used the Master's devouring of good magic against him by overloading him and causing his destruction.