High Mage

The Entity is the sole survivor of a mysterious race who live in an underground city. Using the power of his crystal amulet, he was able to control earth.

2003 Series
The Entity was a member of an advanced civilization that lived underground. When they discovered life on the surface of the Earth, they built an outpost to study it from, which later became the Turtle Lair. However, they discovered that their race was dying. Eventually, the Entity was the only one left.

When the Turtles and the Foot clans mutant experiments enter his abandoned city, he captures them to turn them human again. However, in return, he demanded that everyone he cured had to stay as well as the Turtles. When the Turtles declined his invitation, he tried to keep them there forcibly. The Turtles were forced to use a light orb on him and encase him in crystal.