Like other rulers, he performs one action after every action made by any XCOM squad member, during a special "Ruler Reaction" phase.
- He can use Tongue Pull like other Vipers, binding a soldier with a fifty percent chance to knock them unconscious with every tick.
- As others Vipers, he can also use an area-of-effect breath ability but instead of poison, he spits out a cloud of gas which freezes units caught in it for two turns.
- Like other Vipers, he is immune to poison.
- He will summon a psionic rift after a few turns, or after taking too much damage, and attempt to escape. If he does so, XCOM will encounter him again in a future mission, with the remaining health he had when he escaped.
- Completing the Viper King Autopsy unlocks the Serpent Suit, an armor based on his appearance and abilities.
- He is the first and only male Viper encountered.
- Once killed, the description mentions that he was already reproducing with the female Vipers, despite being only recently hatched.
- There is a glitch where if you kill the Viper King the mission you encounter him, you might not get the corpse. This may vary by platform.
- Ending turns will not trigger the defrost tick on Alien rulers. This makes it possible to completely shut down a ruler by only attacking with Lightning Hands and ending turns to refresh the cooldown.
- In the (pre-WotC) Alien Hunters mission where the Viper King and its brethren are revealed to have escaped, there are (what appear to be) terrestrial snakes crawling all around the map, though they are purely cosmetic (Central even references Indiana Jones' fear of snakes). Given that we are told that Vipers are in no way related to our snakes, this would seem strange; it is possible that Vahlen spliced Viper DNA with that of earthly reptiles to create the Viper King and that these are merely the leftovers from her experiments.
- One could also argue that they are not terrestrial snakes but the hatchlings from the King's nest, but given the size difference this would imply that Vipers grow and mature at a rate that could or would lead to unrealistically (even comically) large snakes; perhaps the Elders engineered them to only grow to a certain size (indeed, they were re-engineered into Thin Men, so it is not implausible that the Elders conducted even more extensive DNA-tampering)