As erinyes are both lawful and major demons, they are not eligible for random generation either inside or outside Gehennom, nor are they an eligible polymorph form for you or monsters. Erinyes would appear in small groups were they capable of random generation.
Erinyes are only generated in certain specific circumstances:
- They can appear in graveyards, including those within Gehennom.
- They can 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.
- The random & in Asmodeus' Lair can include erinyes.
- One can be summoned via the summon nasties spell when cast by spellcasting monsters that are lawful (usually a titan) or in Gehennom (e.g. the Wizard of Yendor).
- They are one of the minions Moloch may send after you if you attempt to convert an altar of his outside Gehennom.
- One is guaranteed to appear in Moloch's Sanctum, if they're not extinct by then.
Erinyes are generally not a threat by the time they are encountered - they are as fast as a player character is normally, but most players will have poison resistance if not a source of speed, and their weapon attack isn't particularly strong. Erinyes are also far weaker than most other major demons, and as such are usually trivial to dispatch.
The erinys is introduced in NetHack 3.0.0.
Per the encyclopedia entry, the Erinyes (more commonly known as the Furies) were female chthonic deities of vengeance that appeared in ancient Greek religion and folklore - the Romans themselves referred to them as "Furiae" in their earthly incarnations. The Erinyes play a major part in the "Oresteia" trilogy of tragedies, especially their themes of justice and revenge.
The Erinyes were commonly associated with night and darkness, and dwelled in Erebus where they heard complaints of insolence brought to them by mortals, punishing such crimes by hounding the culprits relentlessly. They were depicted as crones, with varying sources giving them various traits such as snakes for hair, dog's heads, coal black bodies, bat's wings, and blood-shot eyes. While their number is usually left indeterminate, they were often described as three maiden goddesses - Virgil in particular would recognize three named Erinyes, which would be carried on into modern literature (e.g., the Inferno of Dante Aligheri).
These female-seeming devils named after the Furies of mythology
attack hand to hand and poison their unwary victims as well.
- makemon.c in NetHack 3.6.1, line 1399: Monster birth limit.