The Giant Borg was constructed by the alien inhabitants of a parallel universe to be the ultimate weapon. For reasons unknown, the designers folded it into pocket dimension inside an artifact called "The Giant's Hand" and then hid that within The Neutral Zone, a colorless void bordering The Cosmic Interstate. At some point before he set out on his quest to conquer the Multiverse, Robo-Robotnik v.1 came across information regarding its existence and began trying to pinpoint its location. Robotnik Prime
, who had allied himself with the Freedom Fighters
and the various Sonics recruited by Sonic Prime
from across The Multiverse in order to stop his counterpart, came to the realization that Robo-Robotnik v.1 was seeking The Giant's Hand and revealed its existence as well as location during a meeting with his temporary allies.
Unbeknownst to Robotnik Prime and the others, Robo-Robotnik v.1 had been observing the entire meeting and brought Evil Sonic
to his reality in order to enlist his aid in claiming The Giant's Hand.
When Sonic Prime and the others found The Giant's Hand, Evil Sonic took it from them and along with an army of ShadowBots, distracted all of his counterparts long enough for Robo-Robotnik v.1 to teleport it back to his universe and unlock its power. With The Giant Borg at his command, Robo-Robotnik v.1 set out on a rampage across Mobius Prime only to be defeated and torn apart by the army of Sonics.
With Robo-Robotnik v.1 utterly defeated, Sonic Prime distributed a piece of The Giant Borg (which had turned into a pile of large, metallic jigsaw puzzle pieces) to each Sonic who participated in the fight so that no one could ever rebuild The Giant Borg.
Unfortunately for King Sonic and his family from a possible future timeline, a meeting between a displaced Robotnik Prime and Robo-Robotnik v.2 or Eggman as he is known today lead to Robo-Robotnik v.2's discovery of The Giant Borg piece his Sonic had hidden away in their reality.
Robo-Robotnik v.2 used it to create a new body and unleashed a nuclear holocaust upon Mobius, killing King Sonic and his family along with everyone else. Sometime after he secretly made his way to Mobius Prime, Robo-Robotnik v.2 hired Evil Sonic to pose as Sonic Prime and collect every piece of The Giant Borg and bring it back to him.
Unfortunately, while in the Sonic Underground universe, Evil Sonic mistook that realities Robotnik for his employer (who was wearing a flesh skin suit over his robotic form at the time) and handed The Giant Borg over to him. Robo-Robotnik v.2 decided to give up on acquiring The Giant Borg as he had wasted too much time and resources on the endeavor already and returned to The Prime Universe.
Meanwhile, The Sonic Underground version of Robotnik reassembled The Giant Borg and launched an all-out attack upon Robotropolis, but like Robo-Robotnik v.1 before him, Robotnik was defeated and The Giant Borg was completely obliterated, forever ending the grave threat it once posed the safety of The Multiverse.