A Brute Guard is a specialized Infested Marine used by the Succubus Brood as elite guardians.
A Brute Guard is created when a Terran Marine is infected with the Zerg hyper-evolutionary virus. The victim usually loses most of their intelligence and sanity, as their mind becomes consumed by the Zerg. Most become stronger, faster, tougher, and develop the ability to burrow, as well as other typical Zerg traits (the ability to quickly heal wounds, psionic sensitivity, and so forth).
In appearance, a Brute Guard looks "as if it had been redesigned by a madman who had too many spare parts left over from a variety of species." Growths and tentacles extrude from the body, with ruptured, sunken-in features and festering skin, albeit covered with carapace.