Creatures in The Half-Life Series - Leech

Leech

Leeches are worm-like parasites that survive only underwater. They are very small but aggressive and try to take a bite out of a human. Once a leech is out of the water it can barely move. By one hit from a crowbar it will immediately die.

It is never revealed that Leeches are creatures from Xen. They are often found in floods in Black Mesa in the original "Half-Life". In Half-Life 2 and Half-Life 2: Lost Coast leeches make an appearance as a stage hazard to keep players from traveling too far from the mainland in the coastal areas. The Leeches will appear in large swarms and continually attack the player until they return to the mainland or are killed. Leeches are a sign of the poor condition Earth is in after the Combine took over. Most of the ocean is full of large swarms of leeches and few, if any, fish or other marine wildlife.