ESRB rating

NetHack is a non-commercial game with no ESRB rating, but be warned: it contains sex, drugs, and rock and rolling boulder traps.

The rating it might attain depends on the quality of your imagination: on the surface, it has no graphic violence (indeed no graphic anything), but gameplay generally involves slaughter on a massive scale, murder, theft, genocide, demon sex, hallucinogenic drug use and Altar|occult rituals.