The Viralisk evolved from the Nathrate of the planet Grus, an armored ambush predator that swallowed its food whole. The Nathrate was assimilated and engineered by the Empusae to be the ultimate mobile infestation assault troop for the Succubus Brood. Due to the failed invasion of the planet however, the Nathrate was not completely assimilated and therefore can still be found on its native world.
Viralisks resemble huge green chrysalises that stride along using spider-like legs, and use this facade to disguise themselves as Zerg Eggs. They employ the use of concentrated living chemicals within their bodies by "swallowing" units whole in order for the liquids to infest the devoured target.
Although seemingly unwieldy, the Viralisk can scurry relatively fast for it's size.