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-appearing 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.
- 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.
- 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
- 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 should be able to break the bind by disorienting the viper. (requires confirmation)
- 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 can do increased damage if the soldier is caught for too long (requires confirmation).
- A soldier wearing a Stasis Vest can drastically survive being bound for longer, 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.
- Viper is immune to poison.
- If Viper bounds mind-controlled ADVENT trooper, he won't be released after mind control is lost, while still taking damage.
- 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.
- 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.
- 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.
- Despite the Vipers being female, their previous incarnations, the Snakeman and the Thin Man, were both depicted as male.
- 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.