Kathulos is an Old One, who existed as a vegetation covered planetoid. He could fire powerful mystical bolts, mesmerize others, and fight its enemies in the Astral Plane. Those it entranced risked having their life force absorbed. It was able to control and physically manipulate the vegation on the planetoid.
It's weakness was the fact that a giant Venus Fly Trap-like plant contained its life force. Which like any other plant could be injured. When he brought Dr. Strange to the planetoid and battled him, Dr. Strange discovered his weakness and upon ripping out its roots, Kathulos died.