He is a fusion clone of both Sonic and Shadow the Hedgehog. He was created by Dr. Ivo "Eggman" Robotnik in his laboratory with the Chaos Emeralds, combining their negative chaos energy with the DNA of Sonic and Shadow. He is the completed work of his Grandfather, Gerald Robotnik.

Seelkadoom has the personality combined of Sonic and Shadow, only evil and more destructive. He also appears to have his own personality, likely because of his negative Chaos Energy side.