The Master is the creator of the universe, and GWAR's ultimate overlord.
The Master created GWAR and the entire Scumdog Legion, sending them out on tasks to slowly take over the universe.
When the group led by Oderus Urungus accidentally destroyed Flab Quarv 7 and not Flab Quarv 6 as they were instructed, and revolted against The Master, they were banished to Earth. At first their sentence wasn't so bad, they slaughtered the dinosaurs and built several monuments and generally tried to tolerate the insignificant planet Earth. It was after coupling with apes (and ultimately creating the human race), The Master saw that this was not enough punishment and had Gwar frozen in Antarctica after they wrecked humanity's civilizations.
When they were unfrozen much later, The Master sent many of his minions such as the Destructos with the following ultimatum for Gwar: Return to the Scumdog Legions or be destroyed.
The Master is finally destroyed by GWAR and Scroda Moon when Slymenstra Hymen flushed her tampon (a dead sheep on a rope) down Scroda Moon's portal potty, causing a rift in the space/time continuum and bringing the Master to Earth (We Kill Everything)
Outside of Storyline
The Master's large, fleshy self was often seen fighting Scroda Moon during GWAR's We Kill Everything tour.