The tempest machine is twice as tall as a standard fighting machine, with 4 legs instead of 3 - making it a quad pod. It may be that the Martians intended to build them to wipe out humanity faster but died before they could construct them.
The Tempest is one of - if not the most - powerful Martian machine available in the game. In order to unlock it, the player needs to research Level 2 Fighting Machines to design them, while there must be an Explosives Plant in the zone in order to build them.
The Tempest appears in the game as a boss at the end of Level 7, where it battles the HMS Thunder Child. Unlike in the 1998 game where the Tempest's heat rays appear as a beam of light similar to the standard Fighting Machines, the Tempest's Heat Rays fire Green energy spheres.