Rats of all sorts are common, and the giant Sumatran sort are an evil plague in many places such as crypts and dungeons. Their burrows honeycomb many graveyards, where they seek to cheat ghouls of their prizes by tunneling to newly interred corpses.
Giant rats are long-tailed rodents with a body 12-24 inches long and almost double the length with tail. They are aggressive, omnivorous, and adaptable, and they often carry diseases.
Giant rats are brown/black in color with white underbellies, and are related to the brown rat, with fatter bodies and shorter tails. Giant rats avoid attacking strong parties unless driven by hunger or commanded by creatures such as vampires and wererats. Giant rats are fearful of fire and flee from it except if driven by hunger or magic. They are excellent swimmers and can attack in water as well as on land.
Rats normally flee anything bigger than themselves, but a trapped rat will do anything to survive and a pack of starving rats will attack anything in order to feed.
Rat Swarms. Rarely normal rats (hundreds) or giant rats (at least 50) will swarm attacking as a carpet of biting rats doing 2d4 (or 2d6 in the case of giant rats) damage per round to all in the area (no "to hit" rolls necessary). Melee or missile attacks are ineffective but area attacks (oil, spells) and fire attacks (torches) will damage the swarm (2 HD for normal rats or 4 HD if made of giant rats). Creatures damaged by a rat swarm will have a 15% of contracting a serious blood disease unless they save vs. Poison.