The Nydus Worm is an opening to the Nydus Tunnel Network, which can hold up to 255 units. The units are only lost if all openings to the Nydus Tunnel Network are destroyed, including all Nydus Network buildings. The Nydus Worm is created by the Spawn Nydus Worm ability. The Nydus Worm generates Creep in a radius around itself. Unlike most other Zerg buildings, the Nydus Worm does not require Creep to be present to begin being created.
How the Nydus Worm spawns and if it can move has changed several times. It was first a mobile unit spawned from a Larva, then an immobile unit dropped by the Overseer, and finally an immobile building spawned by the Nydus Network.