Not much known about the Great One, however it is known that it consumed a planet named Arburia. it also has worshippors that help it.
The Great One landed on Earth along with its worshippors, sucking Earth's energy. When The Great One was shedding, The Great Ones worshippors gave Ben, Gwen and Max Tennyson bricks to scrub The Great One. After that Ben turned into several aliens to destroy The Great One but it was to indestructible. After that Ben turned into Cannonbolt the same member of the species that belonged to the planet that it consumed. Ben as Cannonbolt tore into The Great Ones eye and destroyed it from the inside killing it for good.