You may first see this creature in a cutscene at the end of the tutorial mission. Apparently the physical manifestation of the alien's psionic/data network, it is an electric yellow female humanoid being, seemingly made entirely of electricity, save for a metallic skull etched with circuitry, and a bit of spinal column.
You will first fight it after you attempt to Skulljack a ADVENT Captain: you may want to make sure that Captain is otherwise the last enemy on the map. It has high health, a high dodge rate (so even point-blank shots may 'graze' it for partial damage), and high accuracy. For the storyline requirement, the best time to fight it is when there's no turn counter or anything keeping pressure on you. In terms of threat level, physical makeup, and plot significance: it is fairly equivalent to the Long War iteration of the Outsider.
It's abilities include:
- Teleportation: The Codex will usually teleport instead of physically moving. This has a better-than-average movement range, doesn't trigger Overwatch, and the Codex can still fire or use other abilities as its second action. This ability means the Codex will almost always get a flanking shot every round, unless it uses another ability.
- Rift: The Codex can summon a psionic rift. This disables weapons of XCOM soldiers in the area (mechanically, it reduces their ammo to zero). The rift will detonate next turn for medium damage to anything still in the area of effect. This usually costs the turn of any soldier caught by it, since reloading and moving out of the area of effect to avoid the damage is a full turn of actions, though there are options that don't rely on the primary weapon, such as a Ranger's Slash, the Sharpshooter's pistol or using an item.
- Clone: Whenever it takes damage from a direct attack, the Codex will clone itself, the new one appearing not too far away with a Teleport; the remaining HP will be divided between the two clones, rounding down (so if it attempts to clone at 1 HP, it's basically a single teleport). Try to hit it with a high damage attack first if you can.
Flashbangs and other causes of Disorientation greatly reduce it's threat, and seem to restrict it from using it's special abilities. After the first successful encounter, they will spawn in regular enemy pods, no longer in response to Skulljacking/Skullmining, occasionally consisting of all three being Codexes (Codi?). In dealing with them: while Explosives are always a good opener to doing all-around damage to a pod, layered Overwatch hits when they scatter will not trigger their Cloning ability until their movement is finished, giving you a chance to kill them in one move. However, this relies upon the other aliens in the pod not taking the Overwatch shots themselves. If it hasn't gone too well, and you find yourself surrounded by a number of low-health Codi, soldiers with chain kill abilities are very valuable: the Ranger's Reaper ability can chain melee attacks, and the Sharpshooter's Face Off ability shoots once at every visible enemy with their pistol: if you're lucky, you can get a clean sweep on all of them. The Stock gun part deals guaranteed damage on a miss, so a soldier equipped with one can be assured to always at least ding it.
After a certain point of plot progression, you will be required to Skulljack a Codex (and in turn, summoning The Avatar, spoilers ahoy). After this, you can freely Skullmine any Codex enemies for an instant kill whenever needed.