The Parasite Unit, also known as the Skulls, were parasite-enhanced super soldiers commanded by XOF commander Skull Face then later Morrigan. Considered to be his most fearsome assets, the Parasite Unit were given high-priority missions that the average soldier couldn't handle, given their supernatural abilities. Marian Hawke first encountered them in Endor in the First War.
Their prowess in combat was first hinted at when they wiped out Kazuhira Miller's unit in minutes before capturing him.
Sometime before the early 1980s, Skull Face discovered an old Diné man under the moniker Code Talker who had discovered a species of parasite from the remains of The End that assimilated with the vocal cords of the host and could be bred to become infectious when the host spoke a specific language. Besides being a potentially destructive weapon, certain strains of the parasite would also bestow upon the host mysterious abilities (if the body accepted the parasite). Skull Face implemented the parasites alongside additional technology to several unknown XOF operatives in order to create the "Parasite Unit".