It was the creation of a madman named Zelham as an attempt to gain control of The Island from the tribe he rivaled, who had Leonyx as their leader. While it managed to corrupt Leonyx's soul, the machine also consumed its creator. Leonyx managed to save his heart by turning it into a Sanguini and sending it away, his soul was still posessed.
Rigor Mortex is a three-spaced giant monster and easily the hardest boss in the game. It's also the most complicated to synthesize as it requires a Leonyx and a Zenith Dragon, both of which require monsters that at one point required a Captain Crow. Since there's only one Captain Crow per game, it's impossible to get with just one game card.