Sentinel

Sentinels (also called Master Sentinels) are gigantic rock guardians employed by the Drakon Empire to protect their lands. One such Sentinel was abandoned by the Drakon Empire thousands of years ago in the Aquatic Ruin Zone. It rose to the surface in the present day following an underwater earthquake and, after learning its masters had deserted the city that it had been built to protect, the Sentinel destroyed itself. Sonic thought nothing of it.

However, years (and several story arcs) later, the interdimensional Drakon Empire hunted Sonic down, holding the hedgehog responsible for 'destroying' their Sentinel. Sonic escaped back to Mobius after proving his innocence in a death match, but the Drakon came calling again, wanting to know the location of the Chaos Emeralds. When Sonic resisted, they unleashed another Master Sentinel, which Sonic was eventually able to destroy.