Only three can ever be generated; erinys tend to become extinct readily, as all three of them will often appear at once. In a game where they have not been rendered extinct yet, one is guaranteed to appear in Moloch's Sanctum.
An erinys can also be gated in by the melee attacks of barbed devils and horned devils, as well as lawful demon princes, i.e. Geryon, Dispater, Baalzebub and Asmodeus. An eriyns is also one of the demons Moloch may send after you for converting any of his altars outside Gehennom.
In SLASH'EM, the card justice can summon an erinys, even if they are extinct.
- makemon.c in NetHack 3.6.1, line 1399: Monster birth limit.
These female-seeming devils named after the Furies of mythology
attack hand to hand and poison their unwary victims as well.