The Underground Zone is an alternate dimension with similarities to Mobius Prime and part of the multiverse. Distinctions of this Underground Zone include this version of Sonic having a completely different family, including two siblings named Sonia and Manic, and all being part of a band with each other and of a royal family.
The Underground Zone has a slightly similar past to the Prime Zone in that its version of Dr. Robotnik seized the city of Mobotropolis shortly after Sonic and his siblings were born and converted it into Robotropolis. He would then keep the wealthy around to fund his factories while he roboticized everyone else.
The monarch at the time was Queen Aleena, the mother of Sonic, Sonia, and Manic. She took her children into hiding after a bounty was placed on them, but after being told a prophesy from the Oracle of Delphius that would claim the lives of her children would depend on her giving them up, she was forced to part ways with her offspring. She left them to various caretakers: Sonic to this Zone's version of Uncle Chuck, Sonia to a kindly aristocrat named Lady Windimere, and Manic to a good-hearted thief named Farrell. All three became accustomed to playing musical instruments and would eventually meet with each other and form a band. The Oracle of Delphius would return, revealing the children’s history to them. After this, Robotnik would capture Chuck, Windimere, and Farrell and roboticize each one of them. The trio would then form a resistance group together and vow to find their mother no matter how long it would take.
During Robo-Robotnik’s attack in the Giant Borg; Zonic the Zone Cop would go and seek assistance from every version of Sonic across the multiverse. Because of the vast amount of Sonics across the numerous Zones, Zonic forgot about the Underground Zone and its local Sonic. For this reason, Underground Sonic never received a piece of the Giant Borg when they were distributed to all the Sonics in the multiverse.
Evil Sonic would come to invade the Sonic Underground dimension during a mission from Robo-Robotnik to obtain all the pieces of the Giant Borg. He then proceeded to interrogate Sonic, Sonia, and Manic about the location of their Giant Borg piece which left the siblings puzzled. After giving up, he would be captured by Sleet and Dingo, two bounty hunters who were employed by the Underground Zone’s version of Robotnik. Evil Sonic mistook the Underground Robotnik for Robo-Robotnik and hand over all the pieces of the Giant Borg from the other Zones to him. Underground Robotnik likewise mistook Evil Sonic for the Underground Sonic and had him sent away for robotization. Robo-Robotnik secretly arrived on the scene to witness these events and abandoned his mission of recovering the Giant Borg and departed.
Meanwhile, during a concert preformed by Sonic, Sonia, and Manic, Prime Sonic and Zonic showed up searching for Evil Sonic. Underground Sonic and his siblings mistook his Prime-self for Evil Sonic they encountered earlier until Prime Sonic and Zonic explained themselves and their mission. Zonic was surprised about his lack of data on the Underground Zone and its variant of Sonic there and would ask for an explanation of the Zone’s history.
It wasn't long until Underground Robotnik stormed the scene in the rebuilt Giant Borg, believing he robotized his version of Sonic and now looking for Sonia and Manik. The two Sonics, Manic, Sonia, and Zonic formulated a plan that required Underground Sonic to lead the Giant Borg away from Robotropolis. The Giant Borg took the opportunity to fire a heat-seeking missile at Underground Sonic. While Underground Robotnik was distracted, Prime Sonic and Zonic would sneak onto the Giant Borg and corner him. Underground Sonic then turned the missile back at the Giant Borg by running towards it, destroying the weapon for good.
Afterwards, Sonia and Manic, followed by the Sonics and Zonic, would sneak past Dingo and Sleet and free Evil Sonic from his cell in Robotnik's prison. Evil Sonic then pointed out that they are just freeing him from one jail to put him in another but then Prime Sonic would also point out that this way he wouldn’t be roboticized. Prime Sonic and Zonic would thank the Underground Sonic, Sonia, and Manic before taking Evil Sonic and departing from the Underground Zone. Sonic, his sister, and his brother were left pondering if they will ever meet Prime Sonic or Zonic ever again.
Sonic the Hedgehog
Manic the Hedgehog
Sonia the Hedgehog
Oracle of Delphius