The Tentacle is an extremely large organism native to the border world Xen. Apparently blind, the Tentacle displays extreme aggression towards any nearby sources of sound. During the Black Mesa Incident; in which a resonance cascade resulted in an invasion of the Black Mesa facility from creatures originating from Xen, several tentacles were seen throughout the facility lashing out against anything that came near. While seen either alone or in clusters, whether they are individual creatures or parts belonging to a single large organism is unclear.
Tentacles were most notably seen in the rocket propulsion silo in Sector D. Three Tentacles emerged from the bottom of the silo and reacted violently against anything they could hear nearby. Using grenades to distract them, Gordon Freeman, a researcher employed at Black Mesa, reactivated the oxygen, fuel and power supplies to the silo. Activating the rocket, Freeman incinerated the three tentacles.
When hunting for prey, Tentacles will generally tap their blades against nearby surfaces, but will viciously strike at anything they presume to be prey (generally anything that makes a sound). On at least one occasion, a Tentacle is seen dragging a Black Mesa scientist into its pit, likely for consumption.
Whether Tentacles are still present on Earth after the Resonance Cascade and the subsequent Combine invasion remains unclear.