Since the original invasion, the role of an infiltration unit became less important, and when infiltrators were needed, the shapeshifting Faceless were a great improvement on the uncanny Thin Men. As such, attempts to hide their true nature ceased, and they were allowed to return to their original, snakelike genome.
Vipers begin to appear around the second month of the game. Successors of the Snakemen (technically snake women, if their impressive busts are any indication), they serve a variety of combat roles, making their actions somewhat unpredictable. They are also the first enemy fielded to benefit from Dodge, which combined with their good HP, leaves them somewhat difficult to take down quickly.
Vipers have three noteworthy abilities, as well as their standard plasma weapon attacks.
|Attempts to pull a single human adjacent to the Viper.|
|Isolates a single enemy in a grapple, damaging them and disabling them until the Viper is disrupted.|
|Spits a cloud of acid that damages over time, as well as reducing the victims aim and mobility.|
Vipers are a dangerous enemy when first encountered, and have no great weaknesses to exploit. Their HP isn't too shabby at the point you encounter them, and while they have no armor, their high Dodge can frustrate your efforts to kill them. Their high movement speed can allow them to get easy flanks and their plasma rifles will grant them one-shot kills if they crit even mid-game. If you guard against flanks, they can still disrupt your troops very effectively with Pull and Poison Spit. Using grenades to destroy a Vipers cover and focusing fire is effective as always, but can be difficult if other high-threat enemies like Stun Lancers are also in the pod demanding your attention. Thankfully, Vipers don't receive any upgrades over time, so teching up will eventually make them trivial.
Note that their Pull and Bind, while it disables your soldier, also effectively disables the Viper. This can in some ways be a blessing in disguise, since the 2 damage per round of Bind is often less damage than the Viper could be doing running around with its rifle. Aliens will never fire on a Bound enemy, so your soldier is safe for the duration. In addition, after the soldier is released from the Bind they can act immediately and there's no "Catch Breath" debuff like there is with Enemy Unknown Seekers, so your soldier is in an excellent position to punish any remaining enemies in the Viper's pod.
While it may take some time to aim, it is usually possible to lob a grenade that will hit a Viper, but will not harm a soldier bound by Viper.