Appearing in Mobile Suit Gundam Seed Destiny Astray and created by ZAFT in CE 71, the Testament is designed for researching the Earth Alliance's Striker Packs and thus can use them. Like the other ZGMF-X units, it also has Neutron Jammer Canceller and is powered by a nuclear reactor. The Testament's name refers to "a covenant between God and humans", the idea being that Coordinators are gods and Naturals are humans. This name epitomizes the madness and hubris of ZAFT under Patrick Zala.
The Testament is never fielded in combat by ZAFT as it is stolen by EA/The Clan from GENESIS Alpha while ZGMF-X11A Regenerate Gundam is away. Following its theft, the suit is used by a special intelligence unit within the EA and given the model number, RGX-00. It is also upgraded with the Variable Phase Shift armor (VPS armor) and the Quantum Computer Virus Transmission System. Due to the VPS armor, Testament turns from gray to white after activation, and then to red during close combat. The virus transmission system originated from the NMS-X07PO Gel Finieto, it allows the Testament to infect the quantum computers of nearby mobile suits and hijack them. Located in the two large antennae on the head's sides, the system is mainly used to interfere with the enemy unit's sensors.
The Testament's fixed armaments consist only of two beam sabers and one CIWS gun. It can also carry the "Trikeros Kai", a large shield that wraps around its right arm and has several weapons including five claws, a beam gun, a retractable blade, as well as storage points for two beam handguns. In the form of the 'Divine Testament Gundam', the suit gains the AQM/E-X05 Divine Striker, a Striker Pack with flight capabilities and can also change into a giant grappling claw. Both the "Trikeros Kai" and the Divine Striker were developed by EA/The Clan specifically for the Testament.
The Testament bears a resemblance to the ZGMF-X12 Gundam Astray Out Frame, as the latter is built from spare parts of the former found in GENESIS Alpha by Junk Guild technician Lowe Guele. Professional MS pilot, Kaite Madigan, eventually acquired the Divine Testament after it was defeated and made two changes to the suit. Firstly, its red VPS coloring was modified with a white cross often seen on Kaite's mobile suits. Secondly, one of the beam handguns was replaced by Kaite's custom handgun.