Xerfilstyx are Baatezu closely tied to the river styx. They mostly stay on Avernus, the first Layer of Baator. They look vaguely Humanoid from the waist and up, but the lower parts of their bodies are shaped like slugs.
They frequently swim in the river Styx and are known for their madness. Its unknown if they're immune to the rivers memory-draining effect or if the madness is some sort of reaction to the polluted waters.
Sources:
Fiend Folio 3rd edition
Fiendish Codex II: Tyrants of the Nine Hells
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