"The exo-skeleton of this creature is extremely tough, but surprisingly vulnerable to explosive ammunition. The brain is well developed, and its cell growth rate very fast. The creature carries twenty eggs which are laid inside other organisms. This creature is a very effective terror weapon."
-From the X-COM Ufopedia
A very effective terror weapon, indeed. The chryssalid was the most terrifying of X-COM's alien menagerie, an intelligent creature that had the uncanny ability to hide in shadows, sneaking up on your forces and killing even your most highly trained soldier in an instant.
But what was more terrifying than the creature's deadly force was the way in which this force was applied. The chryssalid didn't just kill soldiers. It used the body as a host, planting its eggs within the body, causing the victim to momentarily become a walking corpse. Moments later a new, fully formed chryssalid would burst forth from the victim, leaving just a human shell behind.
In this manner, a single chryssalid could easily and quickly take out several members of your team. Making matters worse was that they were fast, and often by the time you saw one it was too late to do anything about it but say good-bye to the comrade who was standing closest to it.
Meeting a chryssalid during a night mission is one of the more terrifying experiences in X-COM, which is saying a great deal for a game that is known for being one of the scariest computer games ever.