In late CE 73, ZAFT creates a new series of advanced mobile suits to succeed their previous generation of Gundams, which were all lost in the previous war. Since the Treaty of Junius bans the military use of the Neutron Jammer Canceller, these new Gundams use conventional battery power instead. However, technological improvements over the last two years mean that battery efficiency is improved, and Phase Shift armor is less power intensive than it had once been. To further improve endurance, ZAFT also introduces the new Deuterion Beam Energy Transfer System, which allows a properly equipped battleship to remotely recharge a mobile suit's battery.
One of these new suits is the ZGMF-X31S Abyss, a transformable unit specializing in underwater combat. In mobile suit mode, the Abyss is armed with a beam lance, a chest-mounted multi-phase beam cannon, several CIWS guns, and a pair of shoulder-mounted shields which each contain 3 smaller beam cannons and 2 shell-firing cannons. In addition, the Abyss could transform into a streamlined mobile armor mode for underwater combat. In this mode, it is armed with a pair of powerful plasma beam cannons and four high-speed torpedo launchers.
The ZGMF-X31S Abyss uses the operating system Generation Unrestricted Network Drive Assault Module.
The Abyss is initially tested at Armory One by Marle Stroud. However, before ZAFT can bring it into active service, Auel Neider of the Earth Alliance's 81st Independent Mobile Battalion steals the mobile suit and escapes with it to the battleship Girty Lue, where it is given the new model number RGX-02. The Abyss is later transferred to Earth and operates from the carrier John Paul Jones. The Abyss sinks the ZAFT submarine Nyiragongo in the Indian Ocean, but is later destroyed by the ZGMF-X56S/γ Blast Impulse in a naval battle off the coast of Crete.
A special ZAFT variant of the Abyss is the ZGMF-X56S/ε Abyss Impulse, which is a modified version of the ZGMF-X56S Impulse with a new Leg Flyer, enabling the Impulse to emulate the function of the Abyss as an amphibious combat unit.
ZGMF-X31S (RGX-02) Abyss
Manufacturer: Integrated Design Bureau
User(s): Zodiac Alliance of Freedom Treaty, Earth Alliance
Unit type: prototype transformable amphibious mobile suit
First Deployment: 2 October, CE 73
Head height: 17.84 meters
Weight: 92.39 metric tons
Special equipment: Phase Shift armor, Deuterion Beam Energy Transfer System
Powerplant: Ultracompact energy battery
Armament: MMI-GAU1717 12.5mm multi-barrel CIWS x1, MMI-GAU25A 20mm multi-barrel CIWS x2, MGX-2235 "Callidus" multi-phase beam cannon x1, MMI-TT101 Mk9 high-speed guided torpedo launcher x4, M68 dual cannon x2, MA-X223E triple beam cannon x2, M107 "Balaena Kai" beam cannon x2, MX-RQB516 beam lance x1
Pilot: Marle Stroud, Auel Neider