The Scrin are a fictional extra-terrestrial race of intelligent beings in the Command & Conquer universe. Despite being new as a playable side, their origins as a faction go back throughout the Command & Conquer: Tiberian series. Little is known of them, except that they are believed to be responsible for the arrival of Tiberium on Earth and have come to Earth with the intention of mining it. In Command and Conquer 3: Tiberium Wars the Scrin have become a major part of the game, as well as a playable faction.
In the Tiberian Sun GDI campaign, GDI forces located and seized a massive alien vessel of unknown origin that was in the hands of the Brotherhood of Nod, identified as a "Scrin ship". Shortly afterward Nod forces tried to destroy the Scrin vessel to prevent any alien technology from falling into GDI hands. Another device of extraterrestrial origin is the Tacitus, a crystal-like orb that contains vast information on Tiberium as well as a "warning" regarding the Scrin invasion that would later occur in Tiberium Wars. Many of Kane's Tiberium experiments are based on knowledge gained from the Tacitus. In the Firestorm expansion campaign, GDI regards the Tacitus as their last chance at reversing Tiberium's spread across the planet and preventing the world from becoming inhospitable to human life. "CABAL", Nod's AI computer, seems to have decoded at least a part of the Tacitus. The final cutscene in the GDI campaign in Firestorm shows that when the two pieces of the Tacitus were placed together, they merged and the Tacitus was then easily decoded, yielding enormous amounts of data and mathematical formulae.
Information gleaned from Intelligence Database files obtainable during the GDI campaign for Tiberium Wars indicates that the Tacitus may not be a Scrin artifact, but belonging to another alien race related to them.
The formal introduction of the Scrin occurs in the third game in the series. Revealed not as a race using Tiberium to xenoform Earth, the Scrin are instead portrayed as only being interested on the harvesting of Tiberium itself, which they call Ichor. Directly overseeing the operation for Earth is Foreman 371, the player. The player's assistant is the AI of the Mothership the Foreman is based on.
Attracted to Earth due to a massive liquid Tiberium ("Liquid-T" or "Ichor LQ" in their terminology) explosion, the Scrin initiate standard harvesting procedures, deploying ships to the planet surface. To their surprise, Earth is not in the condition they expected: Tiberium levels are premature, the human race is not on the brink of extinction and are still organized enough to be a serious military threat.
The Scrin's method of Tiberium harvesting seems to be as follows: Seed a world with it, wait until the levels are high enough to cause a natural liquid-T explosion, and then begin the actual harvesting by constructing massive "Threshold towers" which mine and send Tiberium to an interstellar location, the "Ichor Hub". It is assumed that by this time, any remaining indigenous forces will be too minor to pose a serious threat.
The first sign that something is wrong occurs when their ships are attacked by GDI's ion cannon network, prompting evasive action. When the AI recommends harvesting be aborted after the threat level is found too high, the Supervisor appears for the first time and overrides the recommendation, ordering diversionary attacks on population centers to lure them away from more critical operations. He also orders harvesting operations and construction of the "Relay Node" and "Threshold" structures to begin as planned.
Information-gathering missions reveal that the liquid-T explosion was not natural, but rather deliberately staged to trick them into arriving. What is truly surprising however is when the person who orchestrated this is revealed to be Kane: The Supervisor exclaims that "this being [Kane] already exists in the data core, genetic derivations are known", implying that they somehow already knew of him prior to the events of the game, but that the nature of Kane's genetic material is not being recognized even by their ancient and vast knowledge. This alludes to a possible reason for Kane's apparent immortality and vast knowledge.
The Supervisor orders more information gathering to be done, but the Motherships AI strongly protests since this will lead to stranding the Foreman on Earth. Eventually, the AI forcibly cuts the Supervisor's communications link. Priority is diverted to ensuring at least one Threshold structure reaches completion, since they contain the means to evacuate the player. This is done and the player makes it to the safety of the 'Ichor Hub' with their ship. Meanwhile, a jittery Supervisor is reporting to the Overlord, explaining the situation. The Overlord orders a full invasion force to be assembled, promising that "Earth. Will. Fall."
An interesting fact regarding the Scrin is that they thrive in Tiberium-based Red Zones. Unlike humans, Tiberium has a regenerative effect on the Scrin, who spray the compound on their infantry and units. Whether they possess a Tiberium-based biology remains to be seen, but it is stated during GDI's campaign that they use Tiberium to both power and construct their vehicles and buildings.
The Scrin within the campaign appear, as a culture, to be a harvester force who have journeyed to Earth in order to gather the 'Ichor', which is their name for Tiberium and may refer to the mineral blood of ancient Greek gods.
The Scrin equivalent of Nod or GDI "Commanders" are referred to as "Foremen" who report to entities known as "Supervisors," who in turn report to an entity known as the "Overlord." Foremen come with an assistant that fills in the role of the EVA units used by Nod and GDI, which like EVA appears to be an artificial intelligence: When it strongly objected to his given orders, the Supervisor demanded that the Foreman "reformat [his] ship."
Scrin units appear to be either living entities or extremely advanced biomechanisms, which generally feature an insect-like design and form. The basic Scrin infantry forces include "Buzzers", swarms of small blades "seemingly guided by a limited sentience" that tear apart infantry in seconds. The effectiveness of rifle rounds on these entities is questionable; the most notable reason for the effectiveness is that the rounds disrupt the 'mind' in the center of the swarm until it dissipates. Other types of infantry are "Disintegrators", specialized anti-vehicle troops that shoot photon beams, and a Scrin engineer-type unit called an "Assimilator"; which is stealthed when not moving and performs the same functions as the engineering units of the human factions. Their heavy infantry are "Shock Troopers", which are a match for the GDI Zone Troopers and Nod's Black Hand Units. The Scrin also possess an elite "Mastermind" unit, which can mind-control a single enemy unit or building in a manner very similar to the Yuri Prime unit from Command & Conquer: Yuri's Revenge.
The Scrin's anti-infantry support includes large insect-like units known as Gun Walkers, equipped with a beam cannon, and Corrupters, which spray liquefied Tiberium. Armored vehicles include Seeker tanks, which fire plasma discs over a wide area, and Devourer hovering tanks, which can enhance their proton cannons by 25%-50% by consuming Tiberium. The Scrin equivalent of the GDI Juggernaut and the Nod Avatar is the Annihilator Tripod, a deadly three-legged walker armed with three laser cannons mounted on flexible tentacles, which can either concentrate fire for more damage or attack up to 3 different targets.
Though they possess capable ground based forces, the Scrin's primary advantage appears to lie in its aerial forces which includes vehicles such as Stormrider fighters that aid Devastator warships. These two are further supported by Planetary Assault Carriers which make use of drone fighters to swarm enemy positions. The Scrin also employ a devastating weapon that can be brought on the battlefield in the form of enormous "Motherships" which are capable of annihilating many ground structures at once in a destructive chain reaction that can "level entire cities".
The Rift Generator is the Scrin superweapon, a device capable of opening a space-time portal (with an event horizon much like a black hole). It cuts a hole in the fabric of space and ejects anything and everything in its vicinity into deep space.
Scrin technology is highly advanced compared to the state of human technology, and in many ways defies conventional physics as understood by humanity. For instance, Scrin weapons technology is based around powerful directed-energy beams that lance out against enemy forces. Plasma weaponry is seen in the form of plasma disc launchers that are used on their Devastator warships and the Seeker vehicles, as well as by Shock Troopers. They have also harnessed the ability to create ion storms (large, persistent static electromagnetic discharges) at will. Scrin aircraft and most vehicles have some type of anti-gravity technology, allowing them to remain suspended over the surface of the planet without apparently ejecting matter or energy, and without acceleration. They also possess the capability to manifest force fields, which appear as projected, semi-luminescent shimmering bubbles which encapsulate units, augmenting their capability in battle. This can also absorb one EMP blast. In addition to force fields, Scrin units and structures may also be protected through use of a 'phasing effect'. The Phase field has the property of partially transferring all objects within its range into another, parallel dimension, therefore rendering them partially or completely immune to attack (but rendering the said units unable to attack themselves). This technology can be used in the form of a temporary field projected at a distance, or through the use of Phase Generators. The latter method is utilized in the protection of the Thresholds until completion, at which point they would become completely immune to "any form of geologic upheaval, severe weather phenomena, cometary impact, or use of conventional, nuclear, Tiberium, or other forms of weaponry", again by an effect described as 'phasing out'. A passing reference is also made of phasing in Tiberian Sun, where Kane mentions "invulnerable flying ships".
In addition to this, they are capable of using wormhole technology allowing them to teleport their forces from one location to another. This is seen by the fact that many Scrin forces on Earth are teleported there from a distant mothership on the threshold of the solar system to the planet's surface through portals. They are capable of using this to transport forces around the battlefield itself and Scrin units such as their Shock Troopers possess 'blink packs' allowing them to teleport short distances away. This has been further developed as a weapon in the form of the Rift Generator, which "cuts a hole in the fabric of space that sucks up anything and everything in its vicinity" and ejects the sucked-up objects into deep space.