The Codex is a mysterious mechanical alien taking on the appearance of a human female with completely golden skin, with a featureless face save for a pair of glowing, white eyes, and a black, smoke-like substance flowing out of its head resembling hair. Its body is in a state of continuous flux, constantly appearing and disappearing on the spot, and sometimes creating short-lived, translucent after-images of itself beside it. The Codex is also seen carrying a standard issue Beam Rifle as its primary weapon.
- Teleport: The Codex is capable of teleportation, moving around the battlefield by moving from one point to another instantly. This ability allows the Codex to completely ignore Overwatch shots, and position itself in ideal firing spots.
- Clone: If the Codex takes a non-fatal hit, it will leave behind a clone of itself in place as it teleports to a new location. Any remaining hit points that the Codex has after being hit will be split between the clone and the original. There is no hard limit to the number of clones the Codex and its clones can create. The Codex is unable to clone itself if it has only 1 hit point left after taking damage, and will simply teleport to a new location instead.
- When hit by an Overwatch shot, the Clone ability is triggered only when the Codex finishes its move. This means you can kill a Codex with multiple Overwatch shots before it has the opportunity to use Clone.
- Codices produced from the Clone ability are exact replicas of the original Codex; they are also able to use Clone and Teleport.
- Psionic Rift: The Codex has an ability to cast a psionic rift that covers a large area of effect, which will disable the use of normal primary weapons inside it, resulting in the weapons being emptied of ammunition and thus forcing players to reload to use them again. The three Chosen primary weapons are immune to Psionic Rift. Secondary weapons like swords and pistols are unaffected. This rift will detonate after a turn, dealing damage to any units caught inside its radius. Any active clones will also have the ability to cast this skill independently of the original.
- While the Codex is a mechanical unit, susceptible to EMP Grenades and bonus damage from Bluescreen Rounds, it is also affected by mental effects such as Insanity and Mind Control as if it were a biological unit. However, Codices have high Will, making this hard to exploit.
- Because of the Clone ability, it is usually preferable to try and destroy the Codex with a single hit. The Ranger's high critical hit chance, or one with the Conceal and Shadowstrike skills, is ideal for this. Bluescreen Rounds (particularly if they are combined with high impact weapons such as shotguns or sniper rifles) and other abilities specialized for mechanical units are highly effective against Codex units as well.
- A Codex will very rarely move on its own, instead using Teleport, which has no cooldown, to traverse the map. The Codex cannot teleport when using its retaliation move when spotted.
- You can prevent Cloning by disorienting the Codex before attacking, Usually done via flashbang. Burning/incendiary grenade does not work because it is immune to burning, however, being a mechanical enemy it can be stunned with an EMP grenade for a short period of time, in addition to the damage the grenade causes.
- A mind controlled Codex does not use Clone when hit.
- Another ideal tactic is to first destroy the Codex's cover without damaging it by using a well-placed grenade or Demolition, and then hitting it with a single powerful shot, like the Sharpshooter's Deadeye. If the Codex survives, both it and its clone will be left with minimal health and the original will be exposed.
- If the Codex does manage to create a clone, a class that can deal damage multiple times a turn is useful for eliminating both units. A Sharpshooter specialized in the Gunslinger tree is optimal, as the clones have very little health and can often be killed with a single pistol shot.
- If the clones have teleported far away, the Sharpshooter's Serial ability can also be used for taking down every clone.
- The Ranger also has the ability to take down multiple clones in a single turn with the Reaper ability.
- The file name of the Codex is "Cyberus", perhaps indicating that the name was changed at some point during the development process.
- The Codex first appears as an enemy unit when XCOM deploys the Skulljack against an ADVENT Officer, following which it will spawn as a hostile unit. On subsequent missions it is usually in pairs of two.