Carnivorous Leech

The Carnivorous Leech (also referred to as a Hagworm) is an aquatic alien creature from the science fiction computer games Half-Life and Half-Life 2.

Description
The Carnivorous Leech is approximately twenty centimeters (eight inches) in length, somewhat snake-like and beige in color. The creature's head is featureless save for a round mouth bristling with sharp teeth. In Half-Life, these Leeches seem to live in fresh water, as they appear in several areas of Black Mesa which have been flooded. (In at least one case, when a room is drained, the Leeches flop harmlessly on the floor for several seconds until they die.) Although they deal relatively little damage and can be killed easily with a crowbar (for example; explosives are effective as well), they can quickly overwhelm the player when appearing in large numbers.



Half-Life 2
In Half-Life 2, the Leeches are found in the ocean along Highway 17, the coast, and the Lost Coast level. A minor nuisance in Half-Life, these versions are impossible to kill, instantly swarming around the player once he swims too far from shore, draining health and armor at alarming rates. As soon as the player returns back to the proximity of the shore, the Leeches immediately cease attacking. In this way, the Leeches are used as a device to prevent the player from leaving the game area, or avoiding obstacles. This is comparable to the existence of the Grue, a creature from the Zork series whose purpose was to prevent players from exploring unlit areas without a light source.

It should also be noted that, upon completion of Lost Coast, the Fisherman will congratulate and invite the player to Saint Olga for feasting, where he states: "I hope you like leeches." It is therefore altogether probable that Xen creatures are now being hunted for food by humans and possibly Vortigaunts, as they are seen cooking a headcrab at one point in the game. Additional dialogue by the fisherman indicates that seagulls eat leeches, too.