Drain spiders are large venomous, mottled brown spiders, about the size of a cat. They make their home in the tunnels below urban areas and are known to be quite aggressive. They feed on trash, bats, and rats, and are considered a great nuisance to city dwellers because of their poison. After a strong downpour, drain spiders can sometimes be forced onto street level in great swarms. These events were christened "spider storms" by the inhabitants of Korvosa in Varisia, although the term has spread to other parts of Avistan, especially around Lake Encarthan.