Gene Worm

The Gene Worm is the final boss of Half-Life: Opposing Force, and the most mysterious of the Race X aliens. This life form is the equivalent of a "biological resource management factory." [1] It's capable of taking any of Earth's natural resources and assimilating it to make them useful to the aliens' needs. If one fully entered Earth and became productive, it would effectively alter the planet enough to become useful as a new homeworld for Race X.

Physically, the creature resembles a green, worm-like creature with many tentacles, with a beak-like protrusion similar in both shape and position to that of the Xen tentacle. The Gene Worm attacks with its tentacles and also ejects a deadly biotoxin stream from its mouth. Its outer hide is impervious to damage; in order to kill it, the player must blind the creature by shooting both its eyes with the mounted laser guns on either side of the worm. This causes it to summon reinforcements by opening its abdominal cavity, revealing a portal orb that a Shock Trooper will teleport through. The player can inflict serious internal damage by firing at the orb.

The Gene Worm is found deep underground in a seemingly abandoned industrial area of Black Mesa, and appears through a portal opening in a room there, apparently either unwilling or unable to fully enter this world. The room itself has been fitted with mounted weapons which are powered by Xen energy crystals. The player character, Adrian Shephard, is sent down to the portal by a Black Mesa security guard, who remarks "Some of your buddies went down earlier, and I haven't seen 'em since." These soldiers were only able to manage to set up the mounted weapons before their unseen demise, although nothing is seen of their remains.