Giygas (known as Gyiyg in Japan) is the main antagonist of Mother and EarthBound (Mother 2 in Japan). Giygas is also known by the names of Embodiment of Evil and Universal Cosmic Destroyer, due to his intent to destroy all reality and condemn reality to infinite darkness. Giygas uses his incredible power to influence evil in all beings around him.
Shigesato Itoi (the creator of the Mother franchise) stated in an interview that his inspiration for Giygas' insane ramblings during the final battle of EarthBound came from a childhood experience. He accidentally stumbled into a movie theater and saw a sex scene in the 1957 Japanese film The Military Policeman and the Dismembered Beauty. Lacking the context of the rest of the movie, young Itoi thought what he was seeing was a rape scene, and it traumatized him and stuck in his mind for years to come.
In Mother, Giygas was raised by Maria and her husband George--a couple abducted from Earth by the alien race that Giygas belongs to. Captured in the early 1900s, George studied the aliens' PSI powers without permission and then proceeded to escape to Earth. Giygas was then ordered to destroy Earth in an attempt to stop PSI from spreading. Giygas, however, did not want to destroy Maria, because she raised him, although he eventually overcame his attachment with her. When he starts the invasion, he is confronted by Ninten and his friends. Ninten is the great-grandson of Maria and George. Once defeated, GIygas promises revenge someday, and then flies away.
In EarthBound, GIygas is once again the main antagonist. His appearance is altered from that of Mother, however. Giygas had gained so much evil power that it destroyed his very being. He became evil incarnate, and is trapped inside of a holding device called the Devil's Machine. Pokey Minch (Ness' neighbor) assists Giygas in the final battle (in which Giygas is incapable of controlling his actions, shown through his attacks that the party cannot comprehend).
In the events of the game, a bee from the future named Buzz Buzz travels back in time to warn Ness of Giygas, how he will conquer the world in the distant future, and to tell him of his destiny to defeat him. Ness and his companions--Paula, Jeff, and Poo--proceed to travel to the location where Giygas is, and then defeat Giygas through Paula's ability of prayer (eventually she has the entire world pray for the party's success; even the player is urged to pray).