Nachash tanninim are a possible inhabitant of a strange-larva-hellish seal. They are primarily notable for their very high physical damage output and 5-square reach. An adjacent target may also be struck by their 1d30-damage slashing shadow, which passes through most forms of armor. The player should therefore seek to destroy them as quickly as possible.
While afflicted with sciaphilia (the love of shadows) the character will suffer from reduced accuracy (-1 per 5 points of sanity lost) and increased spell failure (1 percentage point per point of sanity lost) unless they are standing in a "shadowy" square (a dark square illuminated by a light-source, or a light square darkened by a dark-source).
Tannin (plural tanninim) is an ancient Hebrew word meaning "sea monster" (or sometimes "serpent," and sometimes translated as "dragon"). Ancient Semitic religions used sea monsters to symbolize primordial chaos, and sometimes evil (see also Tiamat/the Chromatic Dragon.
Nachash (more properly spelled nâchâsh) is a Hebrew word meaning "snake."