These are sizable, venomous, mottled brown spiders about the size of a small rat. After a strong downpour, these spiders are often forced onto street level by flooding, boiling up from the drains in great swarms and thus justifying their common name.
AC 21, touch 21, flat-footed 14 (+7 dex, +4 size)
Str 1, Dex 25, Con 6, Int —, Wis 10, Cha 2
Bite—injury; save Fort DC 10; frequency 1/round for 4 rounds; effect 1d2 Strength damage; cure 1 save.