The Master
The true leader of the Underworld who first appears as an eye in a center pool. His true form is a Cthulhu-like demon with a human skull supported by tentacles, which is shown after taking over Matoombo's body. His other eye is embedded in Koragg's shield, and one of his fangs was used to make Morticon's sword and one of his claws were given to Necrolai. Leanbow sealed him in the Pit, ending the Great War but the master was able to transform his foe into Koragg. His minions' mission was to free their Master. Imperious tried giving the Ranger's Legend Warrior powers to him, but Udonna intervened. Koragg stopped him and became Leanbow. The Master was able to partially emerge onto the surface and Leanbow pulled off the ultimate sacrifice, seemingly destroying himself and the Master. Sculpin discovered that Leanbow was somehow preventing the Master's resurrection. Later, Sculpin extracted the Master's spirit from Koragg, giving it to Gekkor for safe keeping. The Master later pealed away from Matoombo's body. The Master then possessed Nick directly and transformed him into a new Koragg, running havoc in the forrest and Rootcore before he was rescued by Leanbow. While the Rangers were distracted by this, the Master went to the Mystic Realm and supposedly destroyed the Mystic Mother. When Leanbow and Daggeron arrived to fight him, he devoured their Knight powers. Following this, he went to Briarwood and hurled the five Rangers into a vision of the future where he had succeeded in devastating the world. Nick was able to wound him and have the Rangers sent back to the real world, but they lost their powers. However, the Master was undone when it was revealed Itassis and Necrolai had defected from him and resurrected Daggeron and Leanbow, the Mystic Mother was still alive and both the people of Briarwood and the forest had gathered their magic against him. Regaining their magic from people's belief in them, all eight members of the Mystic Force overloading the Master with good magic and caused his ultimate destruction.

Morticon Morticon
Mortion is the dark leader of the undead. He is a creature that is part machine, which is powered by moving gears. He is so powerful that it makes it difficult for him to summon his power to the surface world. He uses a mystical portal that allows him to keep an eye on things and communicate with the master. He was ultimately destroyed by the Power Rangers.

Imperious Imperious
Imperious takes over after Morticon is destroyed. He was once known as Calindor who betrayed the Mystics in the great battle. After a battle with Daggeron, he found himself cursed until Necrolai found him in a cave years later. He dared Koragg to give up his magical powers temporarily to prove his worthiness in battle. He challanged Daggeron to a duel, even though he did cheat by teleporting Jenji in his hand when Daggeron was going to send his final blow, he was still destroyed.

Koragg Koragg
Koragg is the Knight Wolf who fights faithfully for the Master, but doesn't bow to anyone else. Although he is evil, Koragg insists that honor is the only way. He controls the powerful dark horse Catastros which is his faithful servant. Koragg has a mental link with Nick, communicating to the ranger through his mind. Koragg discovers his true identity - Leanbow and shortly after is released from the Master's hold.

Necrolai Necrolai
Necrolai is the vicious and unrelentingly Queen of the Vampires, who can fly to and from the surface world through the gate. She decives Koragg from time to time when asked to, such as puncturing him with an arrow and attaching one of Sculpin's scales to track his whereabouts. She has a human daughter named Leelee whose father's identity is unknown. Necrolai might get carried away with pleasing the Master, that she ignores her daughter but when Itassis was going to unleash her wrath, Necorali pleaded her to spare Leelee. She has the power to revive individuals such as Itassis. It was her daughter's confidence in her that caused her to revive Daggeron and Leanbow that caused her to become human.

Hidiacs Hidiacs
Hidiacs are the zombie foot soldiers that serve the evil forces that threaten the planet. They carry axes into battle and beneath their masks is a rotted skull covered in worms. As undead warriors, they fight mindlessly for the will of their undead generals.

Styxoid Styxoid
The Styxoids are a higher evolved form of the Hidiacs. As leaders of the Hidiac army, the Styxoids carry spear like weapons into battle.