Galactus is the sole survivor of the universe that existed prior to the current universe. Originally Galactus was a humanoid named Galan, who was born on the planet Taa, a paradise-like world whose civilization is said to have been the most advanced of any of the known universe of that time.
At the moment Galan's universe met its end, the Phoenix Force amassed the positive emotions of all living beings in the cosmos to preserve them from eternal damnation, enabling the Sentience of the Universe — the previous universe's equivalent to Eternity — to meet with Galan. Within the "Cosmic Egg" the Sentience of the Universe revealed itself to Galan and informed him that though they both would die in the final moments of the universe, they would both survive through a joint heir born into the next universe. The Sentience of the Universe merged itself with the mortal Galan and thus Galactus, the devourer of worlds, was conceived.
After Doctor Doom made a deal with Dormammu to obtain a copy of Ultimate Marvel vs Capcom 3, Dormammu snuck in a note after learning Doom cheated him with Bison Dollars, proclaiming if Doom and Max did not master all twelve new characters within a certain amount of time Galactus would "scorch the Earth". Indeed Galactus appeared at the end of the first episode of Ultimate Assist Me, warning Max and Doom of his approach.
During the final episode, Max confronted Galactus within the cursed disc using Strider, Ghost Rider and Vergil. Max defeated the devourer of worlds and saved the Earth, though the cursed disc vanished soon after.