Shuma-Gorath is the manifestation of the Ancient One's
dark side. The Ancient One is a powerful sorceror, who tutored Dr. Strange in
the ways of the mystic arts. Shuma-Gorath possesses all of the Ancient One's
abilities, and is the embodiment of evil. When he encounters other creatures,
he appears as that being's worst nightmare.
In his dimension, he became the Sorceror Supreme, and thus opposed his
counterpart in our dimension, Dr. Strange. His attempts to defeat Strange
were unsuccessul, but he was all to successful in his own dimension. He
gained great power and destroyed the Chaos Dimension. Now, he travels through
other dimensions, looking for enough power to sate his hunger. He has now
set his eyes on our dimension, and his goal is to devour the Earth and
absorb it's energies.