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UNSC WARNING: This document, REM-225 Eris, is under the auspices of Naval Special Weapons Project EXCALIBUR (2532-2553), classified TOP SECRET / NOVEMBER BLACK (BGX Directive). Access or disclosure of classified material is in violation of the Wartime Articles of Secrecy, and is punishable by imprisonment or execution on charges of treason. ONI Seal 1
REM-225 Eris
Production information

Bellum Aerospace Industries




Electronic Warfare drone

Technical specifications
Damage Per Hit


Ammunition Type
  • BLINDER counter-ECM system
  • SPARTOI decoy system
  • Ship-launched
Rate of Fire

Launcher capable of firing rates of up to sixty launches per minute.


Extremely high (computer guided)

  • Standard: Four hundred thousand kilometres
  • Two-stage MSM: One-point-two million kilometres at continuous maximum power
  • Three-stage MSM: Two million kilometres at continuous maximum power (all ranges specified are minimum figures)
  • Pulse Laser Turrets



The REM-225 Eris is an electronic warfare missile developed by the UNSC Navy to counter superior Covenant EW systems during the First Great War, but only saw its first use in the War of Vengeance.


Development of ship-launched missiles stalled after the age of space colonisation - the creation of a comparable navy required resources and infrastructure that simply wasn't available to mere pirates, privateers and insurrectionists, allowing the Navy almost two hundred years of naval dominance. When the Covenant attacked, the UNSC found itself in the unenviable position of having to field a fleet intended to support ground campaigns unhindered against a technologically superior fleet of warships. Progress was made even in the early years of the war, but in the end it all came down to a matter of technology - the Covenant had the advantage, and the UNSC wouldn't even come close to catching up for decades.

As part of Project TEMERITY, the Office of Naval Intelligence investigated a range of replacements and supplements to the standard Archer Anti-ship Missile, eventually adopting a number of different types to fill extremely specialised roles in ship-to-ship warfare. The result was missiles such as the RIM-442 Anubis, the RIM-441 Neith, and the RIM-332 Azrael, each designed almost exclusively to deliver significant damage to unprotected ship hulls, relying on more "powerful" nuclear or EMP warheads, or MAC fire, to lower these shields first, leaving the ship vulnerable. Once past this defensive layer, they proved devastating, and would see wide adoption throughout the Navy after the end of the First Great War. By the time of the War of Vengeance these had become standard ship armaments, though ONI had restarted Project TEMERITY to devise improvements to the weapon system.

Despite these immense improvements, however, the question of the Covenant's superior point-defence and ECM systems remained. Covenant point-defence systems were capable of destroying up to half of the missiles before they reached their targets, and jamming systems were capable of sending them on wildly different courses away from the target. The UNSC's answer to this was the REM-225 Eris missile, an electronic warfare drone equipped with two revolutionary systems, BLINDER and SPARTOI.

The first block developed were intended to be used in conjunction with the RIM-332 Azrael nuclear missile and the RIM-441 Neith fragmentation missile against heavily shielded and armoured Covenant ships. Camouflaged by the other missiles' efflux streams, upon entering range, the Eris missile would speed ahead of the rest of the salvo and activate its BLINDER system, blocking defending ships' anti-missile systems with strobing jamming, rendering enemy fire control almost useless. Taking advantage of the gap in the electronic defence, the missiles would enter their attack range. the Eris' SPARTOI system would then deploy, which presented hundreds of false profiles to act as decoys for anti-missile systems targeting standard missiles. They pretended to be an entire volley of missiles, attracting point-defence fire that would otherwise be targeted at actual missiles.

The second block of missiles was equipped with an enhanced EW system, and was also an MSM, and so had double or triple the range of a standard single-stage missile. This extreme extension of range meant that UNSC ships could strike at Covenant vessels from far outside plasma torpedo range. Along with the arsenal ships they proved to be "war-winners" for the UEG during the War of Vengeance, and also saw service against the Governors of Contrition during the Second Great War.


The first block of missiles developed consisted of one electronic warfare computer in the place of the warhead, an advanced Hall Effect Thruster, with smaller chemical rockets along its horizontal axis for three-dimensional thrust vectoring in space, a fusion reactor for power, and in the nose cone, a GGF8 target acquisition computer. Since the missile did not carry a large warhead the designers were able to squeeze in a miniturised hafnium-based fusion reactor, giving the EW system an enormous energy budget from which to draw power for the BLINDER and SPARTOI systems. A smaller, single-stage variation of the missile, the AEM-226 Eris, is used for electronic warfare purposes by the C800 Blackburn Claymore Space Superiority Fighter.

Designed just prior to Operation: FURY, the Mark II missile was designed as a Multi-Stage Missile, or MSM. It was equipped with a second-stage Hall Effect Thruster that took over after the first stage had burned out, effectively increasing the missile's range by a factor of two or more, giving the missile a minimum range of 1.2 million kilometres. Larger models, such as those fired by the Invincible-class, were equipped with three stages, and had minimum ranges of 2 million kilometres. This meant that UNSC ships could now launch strikes on Covenant ships from far outside plasma torpedo range. Since the enemy could not strike back, this effectively made the UNSC Navy almost invincible, until the development of new ship classes with a greater emphasis on energy projectors by the New Covenant.


The BLINDER system was developed as part of Project EXCALIBUR, designed to confuse the primary targeting radars of defending ships' anti-missile systems with strobing multi-frequency jamming. The frequencies are constantly altered in a random non-repeating pattern, programmed into each conventional MSM to avoid jamming their fire control as well. This forces point-defence targeting systems to rely on less effective visual, LIDAR, or infrared tracking. It also attracts jam-home weapons to take them away from conventional missiles. Thanks to the micro-fusion plant on board, the system has a massive amount of energy to throw at enemy sensors.


Named after the warriors in Greek Mythology that sprang from the ground when Dragon's Teeth were sown, the SPARTOI system consists of two hundred ballutes that are ejected from the Eris missile when BLINDER activates. Each ballute contains a radioistope thermal generator, which together with the ballute's lack of stealth features, makes it appear to be a real missile. Each carries a single high-power laser emitter and ultraviolet diode designed to confuse or outright blind LIDAR and ultraviolet point-defence targeting sensors, as well as a multi-frequency jammer designed to confuse point-defence radars and draw jam-home point-defence fire on to it and away from the missiles. They act as decoys for anti-missile systems targeting standard MSM missiles, attracting point-defence fire that would otherwise be targeted at actual missiles. Due to the fact that the false profiles outnumber real missiles by seven-to-one, considerably more false missiles are "destroyed" than real missiles, allowing the maximum number of warheads to hit the target.


"Eris was the Greek goddess of discord. Appropriate, is it not?"
―Anonymous UNSC Navy officer.