|“||Without the introduction of human DNA, these creatures once operating under the guise of "thin men" now show their true form: a purely reptilian species.||”|
–Dr. Tygan upon encountering the first Viper
They first appear in April as pod leaders for ADVENT forces. They start the transition from leader to support troop in May (June on Legend). At this point, pods with multiple Vipers or combinations of Viper + other aliens are possible.
This female reptilian alien is a deadly combination of strength and speed. This is the true form of the Thin Men from XCOM: Enemy Unknown. It is initially assumed that Thin Men had fallen out of use as the aliens did not need infiltration units after Earth's surrender, however this was debunked with the introduction of the Faceless.
Despite their resemblance to terrestrial snakes, the Viper is actually a wholly extraterrestrial species unmodified by human DNA. However, it retains the modified Thin Man's poison and high accuracy, combat abilities and intelligence.
- Bind and Crush: The Viper needs to be adjacent to the target for this to work, and it can still miss (albeit rarely). The target will be bound, unable to take any action, though the Viper will also be unable to shoot or move. Each round, the Viper can use Crush dealing damage to the unit. Vipers can only Bind humanoid targets - mind controlled Vipers can be used to Bind ADVENT soldiers, but not Mutons or MECs. A failed Bind attempt will trigger counterattacks by melee-capable targets.
- Poison Spit: The Viper can spit a poison cloud that deals damage in an area, and also inflicts Poisoned on the target. This reduces Aim and Mobility. This ability has a 1 turn cooldown.
- Tongue Pull: A long range free action ability, that when it hits, deals no damage, but pulls the target out of cover and right next to the Viper. She will then automatically use Bind on the target. It has a 2-turn cooldown.
- Plasma Blaster Shoots the Viper's gun at the enemy.
- Vipers have an unusually high dodge stat, making "graze" shots - dealing reduced damage - more common. Plan accordingly by using explosives or placing more soldiers on Overwatch than normal.
- Like the Seeker from the previous game, a Viper can bind a soldier in a strong strangulating coil. While this is active, neither the Viper or the bound target may perform actions, and the bound target will take damage, eventually becoming Unconscious if they are not killed.
- A "bound" soldier (after a Tongue Pull) can be freed by causing damage to the Viper.
- If carefully positioned, a grenade can be used without hurting the soldier (flashbang also works).
- A Psi Operative can break the bind with Insanity.
- Avoid going near the Viper and try to pick it off at a distance, this helps avoid the Tongue Pull and Bind, be wary of the plasma rifle at range as these can still be very powerful.
- A Ranger with Bladestorm can actually free themselves from the bind, causing them to be stuck in the bind animation. Saving your game and loading it again will fix animation bug.
- Bind will do increased damage if the soldier is caught for too long.
- A soldier wearing a Stasis Vest can dramatically improve the soldier's longevity while bound, giving you more time to free them.
- If a bound soldier is wearing Wraith suit, he can activate "Wraith" and move to adjacent tile freeing himself. (This interaction may be a bug)
- Hellweave vest triggers on Tongue Pull, making attack 100% miss and making the Viper burn.
- If Viper binds a mind-controlled ADVENT trooper, he won't be released after mind control is lost, while still taking damage.
- Vipers are immune to poison.
- If the Viper misses when using Tongue Pull, they are likely to attack with Poison Spit instead, take care not to group your soldiers too close together, or equip them with Hazmat Vests, Medikits, or Nanomedikits.
- If a soldier is infected with poison, they will become contagious and infect any units who stand next to them with poison as well. Unwary positioning can lead to your entire squad becoming poisoned.
- If a soldier discovers a Viper by moving to a tile next to said Viper, the Viper will bind the soldier without needing to wait for the alien turn.
- The Viper has been mentioned by Firaxis developers Garth DeAngelis and Jake Solomon to be the original appearance of the Thin Man before undergoing mass heavy genetic engineering in order to modify them into a more human form for infiltration purposes. This is also mentioned in-game when they are first encountered and during the Viper Autopsy.
- According to its description on the official XCOM 2 website... "With her long tongue, the Viper can pull your units from cover and draw them out into the open. Get close enough to the Viper and she will pounce, wrapping her lengthy reptilian frame around your soldier with her Bind ability..." The Viper is described as female. The same is also described in the Alien Hunters DLC's first mission.
- Despite the Vipers being female, their previous incarnations, the Snakemen and the Thin Men, were both depicted as male, and their ruler is called the Viper King. Though it is explained in Vahlen's notes that the Ethereals seem to have kept Vipers restricted to females only, in order to control the species by only allowing them to reproduce with their help.
- Based on the descriptions of the Battlefield Medicine and Dark Event (Viper Rounds), Viper venom likely contains a combination of neurotoxins, dendrotoxins and hemotoxins, the latter of which are used to help create Nanomedikits.
- While currently not in widespread use, the same principles are currently being researched with real-life snake venoms.