R-4913 and X-8973

X-8973 is a Vortigaunt from the Half-Life: Decay bonus mission "Xen Attacks" that can be unlocked by the player who manages to obtain an A rank on all of the regular missions.

In this mission, both players play as Vortigaunts, "Drone Subjects" X-8973 and R-4913, two Vortigaunts sent to Earth by Nihilanth. Although short on story, the mission gives some unique insight into the Vortigaunts' role in the Black Mesa Incident.

The mission Nihilanth has assigned them is the retrieval of crystals stolen from Xen by the Black Mesa science team. Both Vortigaunts are able to regain health by damaging objects or enemies, supporting the theory that Vortigaunts can drain lifeforce/energy with their beam attacks.