The Chryssalid is an insectoid alien that could be considered one of the most annoying units in the game. Their main form of attack is to inject an embryo into their target which will then turn into a "Zombie" that--if not killed after a set amount of turns--will then hatch into yet another Chryssalid who can then go on to repeat the process all over again. It is not uncommon to find yourself overwhlemed by a seemingly infinite supply of Chryssalids or Chryssalid Zombies in any given scenario. They primarily seem to show up in Terror Missions, where there are plenty of civilians for them to convert to new Chryssalids.
They are also quite fast and have a fair amount of protection against critical hits with their armor; which can be used to make Chitin Armor if you should succeed in killing one. They are also immune to Stun, so they cannot be captured alive.