This transformation exists primarily or exclusively within the Pre-Super Genesis Wave continuity. Information in this article may not be canonical to the storyline of the games or any other Sonic continuity.
The Finalhazard consists essentially of its original form merged with the Space Colony ARK on the Eclipse Cannon, making the entire ARK a part of its body and the Finalhazard one of the largest entities in the series. At the tip point of the Eclipse Cannon, the Finalhazard's organic body can be found, which is its original form. From its hips and down though, the Finalhazard's body has merged with the cannon's metal and has thus lost its hind legs and tail.
When Sonic the Hedgehog and Shadow the Hedgehog arrived in the Space Colony ARK's shrine room, the Biolizard used Chaos Control to teleport away and fused with the ARK's Eclipse Cannon, becoming the Finalhazard. From there, the Finalhazard began pulling the ARK towards Mobius so it could crash into the planet and destroy it. However, before it could fulfill its mission, it came under attack by Super Sonic and Super Shadow, who defeated, and presumably destroyed, it with their combined efforts.
The Finalhazard's design appears to be based off the Final Lizard's design from Sonic X, possessing both forelegs and no hind legs, unlike the Finalhazard from Sonic Adventure 2, who retained its back flippers and only had a stump for a left foreleg.