Sonic: The One Ring is a hack of the original Sonic the Hedgehog game for the Sega Mega Drive / Genesis, created by Mercury. Its defining (and titular) feature is that in each Act, all rings but one have been removed, and Sonic cannot complete the Act without this One Ring in his possession. Thus, in each Act, Sonic must search for the One Ring that will afford him the opportunity to progress. Several alterations have been made to the game, to somewhat balance the lack of rings, and allow backtracking during the search.