Mahabharata 12.83.40

अग्निना तामसं दुर्गं नौभिराप्यं च गम्यतेराजदुर्गावतरणे नोपायं पण्डिता विदुः

agninā tāmasaṃ durgaṃ naubhirāpyaṃ ca gamyaterājadurgāvataraṇe nopāyaṃ paṇḍitā viduḥ

Our plain-language meaning of this verse is waiting for scholar review.