The High Mage is an antagonist in the 2003 Teenage Mutant Ninja Turtles cartoon series. He is the surviving member of an ancient race known as the Y'Lyntians.
The High Mage originated from an advanced civilization who used crystals that stored Cosmic Energy. With this power, he and his kind dominated the Earth and wanted to prove themselves as superior to the humans as they enslaved them and transformed some of them into mutants to make them do heavy labor. However, the human race and their mutant allies rebelled against the Y'Lyntians and caused their demise as they destroyed their largest stone and source of power, the Sun Crystal. The High Mage and a few remainder of his people retreated to an outpost city underground where the surviving members of Y'Lyntians were encased in crystals in order to survive after their crystals had stopped working. The High Mage, however, stood awake to guard them.
One day, the High Mage discovered the turtles and the Foot Clan's experiments. He offered his services to change the Foot Clan experiments back to humans by placing them inside crystal cases, but he tells the turtles he is very lonely and wanted them to stay with him. When the turtles decline, the High Mage became outraged and fought them. The turtles managed to defeat him by trapping him inside a crystal. The turtles felt bad for the High Mage after that as they said he was all alone and not actually evil.
The High Mage and his people were later revived as the planets aligned themselves that restored the crystals' energy as it also repowered a big fragment of the Sun Crystal that was recovered known as the Crystal Moon. After the turtles discovered the crystal they had was glowing, they traveled back to the underground city where they discover it entirely populated with Y'Lyntians. The High Mage was seen enchanting with a group of Y'Lyntians to raise their island home, until he noticed the turtles and threw four red crystals at the lava below, creating four lava monsters to subdue them. The High Mage later appeared before the turtles after they landed in a cell.