Combat Skin is a Forerunner term relating to body armor. The Forerunners had a classification system for combat skins, running from Class 1 to 18, and possibly above. According to 343 Guilty Spark the Human MJOLNIR Mark V is ranked on this scale as equal to a Class 2.
Very little is known about the Forerunner Combat Skin other than that it was used by combat personnel and was comparable to, but far more advanced than the UNSC MJOLNIR Powered Assault Armor.
It is first mentioned by 343 Guilty Spark as he is leading John-117 through the Library, wondering why he had brought "such ineffective weapons" to combat the Flood. He notes that the MJOLNIR is ill-suited to combat the Flood, and recommends that he upgrade to a Class 12 Combat Skin from the MJOLNIR, which is ranked on the Forerunner classification system as a Class 2. On the other hand, Guilty Spark also notes that the MJOLNIR will serve John well when the Flood begins converting Halo's atmosphere, commenting that he is a "good planner". Upon learning that there was a powered armor system six times more powerful than the MJOLNIR, John expressed enthusiasm at the opportunity to wear one.
The Terminals on Installation 00 indicate that other Combat Skin classifications existed, ranging from 1 to 4, and that there were other suits of a similar nature, such as the Battle Harness, with its own classification ranging from 1 to 6, and the Platform Interface Skin. Eventually, all Civilian personnel were required to wear at least a Class 8 Combat Skin unless in core areas, and all Military personnel were required to wear at least a Class 12 Combat Skin.
During at least the early years of the Forerunner-Flood war, Forerunner ground troops used Combat Skins that, while certainly powerful, lacked energy shields, leaving their wearers extremely vulnerable to Flood infection. It is likely that, rather than representing the peak performance of the technology, it is indicative that the Forerunners were woefully unprepared for the Flood onslaught, and were initially slow to adapt their equipment and technology to fighting a purely biological foe.
During the Battle of Installation 00, Guilty Spark can be heard lamenting that John did not upgrade his Combat Skin. The fact that the Mark VI is more advanced than the Mark V that Guilty Spark had previously encountered and is still classed as a Class 2 may indicate that the Combat Skin class system is based on another factor other than combat performance, or that despite its upgrades it was still inferior to a Class 3 Combat Skin.
Combat Skins were used by the Forerunners during their war with Humanity. After Humanity had been beaten and de-evolved by the Forerunners they continued to wear their Combat Skins against their new foe, the Flood.
When awakened by John-117, the Ur-Didact was wearing his Combat Skin.
Because so little is mentioned about Combat Skins, there is much speculation about exactly what the Combat Skin rating indicates. The obvious interpretation is that the Combat Skin ranking system measures raw armor power, and that a Class 12 Combat Skin is 10 ranks higher in protection and power than a Class 2, but if the Forerunners were able to use a suit at least six times more powerful than John's, it's a wonder they had such trouble with the Flood, unless the Flood made use of such 'Combat Skins' themselves, such as an Elite Combat Form using a personal energy shield. This would mean that both forces would be equal in suit strength, as well as weapons and vehicles, while the Flood would most certainly outnumber the Forerunners. The most likely scenario is that in the Forerunner-Flood war, the Flood infestation was billions of times larger in scale, which would mean that despite the Forerunner's near invincible armor, they may simply have been outnumbered. The fact that Forerunner weapons were probably effective against Combat Skins meant another advantage for the Flood.
Another interpretation is that the class system does not measure raw power, but rather classifies armor by its role. Later in the game, as 343 Guilty Spark comments on the Flood's alteration of the atmosphere, he notes that John-117's Environment Suit should serve him well. This may indicate that Combat Skin with a Class 2 designation are designed for use as environment suits in hazardous terrain, rather than as effective armor to be used in combat against the Forerunners' enemies. There is a discrepancy to this assertion, however, as one of Halo 3's terminals makes mention of higher classes of Combat Skin that were vulnerable or ineffective against the Flood.
Another possible permutation of this theory is that the number could perhaps designate the order in which the suits were designed, number 2 being a low-shielding model created by the forerunner to avoid atmospheric hazards possibly for industrial use, which would explain the protocol of civilians wearing suits Class 8 or above (they would have been designed recently enough and have some form of technology sufficient to protect the wearer in the case of an outbreak). This would make the 12-rated suit designed well into the war and specifically created for combat against the Flood, but it wasn't used soon enough to turn the tide of the war.
It is also possible that numbers still represent the power level of each armor, but each design contained a flaw that could not be overcome or, while it was considered "more powerful" than the ones below it, each rating was only designed for specific roles.
Another theory states that a Combat Skin may also be an environment suit. A modern day analogy would be a HAZMAT suit, which is an environment suit, but has no further use in combat, while a suit of Kevlar provides physical protection but no environmental protection. John-117's "Combat Skin" also functions as an "environment suit"; this can overlap with other theories, as a useful environment suit may lack in either pure power or suitability for its role in combat. Quite possibly, John-117's MJOLNIR may be considered half environmental and half battle armor in Forerunner standards.
Armor Ability Stacking
Another possible theory is stacking. If a normal HAZMAT Suit has a ranking of 1E (1 Environment Point) and a high class KEVLAR Suit has a ranking of 1SA (1 Shield & Armor point). Mk VI (as stated above, and shown in game) is a 1E + 1SA, giving it a rank of 2ESA (ESA being a single class rank in the Forerunner system), giving John's Mark VI MJOLNIR a Forerunner rank of Class-2. With this being said, 343 stated that John should wear at least a Class-12 Combat Skin, which would be the equivalent to 6 post-modern HAZMAT Suits and 6 post-modern KEVLAR Suits or another 5 layers of MJOLNIR armor.
- Early concepts of the Forerunners' combat skin bears slight resemblance to the robotic Geth from Mass Effect.
- In the Halo 4 Terminals, Forerunner armor is shown to have golden visors. This could possibly be linked to The Librarians tampering with humanity to eventually create their own Combat Skins, as some MJOLNIR armor notably have golden visors as well.
- Any Forerunner's combat skin can use lighting on their skin to convey the emotions of its wearer.