Pre-Super Genesis Wave continuity. The information contained applies only to that canon.
Ultra Sonic is a super form of Sonic the Hedgehog that has only been attained through using massive amounts of magical power. He first appeared alongside Hyper Tails to battle the evil Ultra Naugus, through using a huge cache of Power Rings created by Nate Morgan, only to disappear as Sonic's power was depleted. Later, when Knothole was threatened by a beam of energy that could have disrupted the entire time space continuum, Sonic used a Super Emerald found in Dr. Morgan's travels to achieve this form in order to race around the planet and prevent the beam from making contact. Not long after, Sonic briefly resumed this form to end a rematch with Ixis Naugus in the Southern Tundra.
When Ixis Naugus raided Nate Morgan's secret tundra lab, he, as well as Sonic and Tails were infused with energies from a large cache of Power Rings. Though Ultra Sonic managed to hold his own against Naugus' attacks through manipulating the elements at the atomic level and with the aide of Hyper Tails, Ultra Naugus seemingly had greater stamina than either and waited for their transcended states to burn out before he attacked Morgan. After Nate sent Naugus back to the Zone of Silence, the two heroes found their Power Ring induced energies depleted, causing them to revert back to normal and in a weakened state. Upon being awakened, they had no memory of their advanced transformations until Nate told them of it. (StH: #65, #66)
Second TransformationWhen one of Dr. Eggman's satellites fired a beam that moved slowly towards Knothole and threatened to disrupt the space-time continuum and destroy the planet itself, Sonic used the energy of a Super Emerald to ramp up his speed in order to run around Mobius at a high enough velocity to stop the effects of the attack. As he ran, Sonic's power level continued to increase until he became first Super Sonic, then Ultra Sonic, and then Ultra Sonic's various alternate forms- Solar, Polar, and Eco Sonic. (StH: #71)
Some time soon after, Sonic returned to the Southern Tundra to place flowers at Eddy's grave in memory of his sacrifice. He fell into the castle's ruins however, and found Naugus had been able to return himself to Mobius from the Zone of Silence and planned to use the remains of Nate Morgan's ring storehouse to regain his transcended state and merge the two zones to make him omnipotent. In the battle that followed, Sonic tricked Naugus into burying him with the stockpile of Power Rings and used them to become Ultra Sonic once more. Desptite having less finesse with this form, he applied his power and speed to open a portal to the Zone of Silence that sucked Naugus back inside and promptly closed. The rings' power then wore off due to how much Ultra Sonic had used up to open the portal, and he resumed his normal state. (SSS: #15, FCDS: #5)
Ultra Sonic looks exactly like Sonic normally does except he is surrounded by an atom cloud which closely resembles a nucleus atom. Ultra Sonic leaves behind tiny trails of stars as he moves.