When Knuckles the Echidna met The Chaotix for the first time and set out to free Sonic and the Freedom Fighters along with hundreds of innocent Mobians who had been imprisoned in a Hall of Mirrors at a fake Carnival constructed by an entrepreneur named Renfield T. Rodent and Robotnik Prime.
When they took the fight to Robotnik and Renfield, they were assaulted by the original Metal Sonic, who would later become the Secret Freedom Fighter known as Shard.
They managed to defeat Metal Sonic but before they could destroy it, Metal Sonic took on a newer and far more powerful form: Titan Metal Sonic. Only by using a "power gem" was Knuckles, who had been de-powered when everyone else was imprisoned, able to defeat Titan Metal Sonic, regain his powers and free everyone from imprisonment.
Project Titan and The Hybrid
Following the reality-altering events of "Genesis", Doctor Eggman unleashed Project Titan, a Titan Metal Sonic robot built from scratch, upon New Mobotropolis.
Unlike it's predecessor, Project Titan was a feral machine whose prime directive was to commit acts of murder and genocide.
Despite their best efforts, the Freedom Fighters were unable to stop Titan Metal Sonic on their own and turned to Ixis Naugus for help.
Using his crystalline magics, Naugus was able to bend Project Titan to his will and use it to shield New Mobotropolis from a massive energy beam that had been fired from the Death Egg Mark II, thus preventing the city's total destruction.
Eggman would later go on to build another Titan Metal Sonic, which he later fused with a reprogrammed Krudzu Hydra Hybrid to fight Team Fighters.
The battle with the Krudzu Titan Metal Sonic Hybrid was rather short however and ended with Sonic forcing it into the ocean, which destroyed Titan Metal Sonic and incapacitated the Krudzu Hydra Hybrid.