Gene Worm

The Gene Worm is a large, tentacled alien life-form that serves as the final boss for the Opposing Force campaign. It strongly resembles the Xen Tentacle in both its color and its beak-like protrusions, meaning that the two must be related.

Background
It invades an abandoned sector in the Black Mesa facility through a dimensional portal, and players must work together to kill the creature before it fully emerges from it. It will attack players with its toxic breath and will lash out with its tentacles, but it can be temporarily blinded using the mounted laser guns on either side of the portal.

Once it is completely blinded, the creature will expose its heart and create a portal that will summon an enemy. The heart is its weak point and must be attacked with the lasers to damage it. This procedure must be repeated enough times until the Gene Worm finally dies.