Mbh 2.30.3a

Vaisampayana said,--‘It is said, O Janamejaya, that the god Agni while residing in Mahishmati, earned the reputation of a lover. King Nila had a daughter who was exceedingly beautiful. She used always to stay near the sacred fire of her father, causing it to blaze up with vigour. And it so happened that king Nila’s fire, even if fanned, would not blaze up till agitated by the gentle breath of that girl’s fair lips. And it was said in King Nila’s palace and in the house of all his subjects that the god Agni desired that beautiful girl for his bride. And it so happened that he was accepted by the girl herself. One day the deity assuming the form of a Brahmana, was happily enjoying the society of the fair one, when he was discovered by the king. And the virtuous king thereupon ordered the Brahmana to be punished according to law. At this the illustrious deity flamed up in wrath. And beholding this, the king wondered much and bent his head low on the ground. And after some time the king bowing low bestowed the daughter of his upon the god Agni, disguised as a Brahmana. And the god Vibhabasu (Agni) accepting that fair-browed daughter of king Nila, became gracious unto that monarch. And Agni, the illustrious gratifier of all desires also asked the monarch to beg a boon of him. And the king begged that his troops might never be struck with panic while engaged in battle. And from that time, O king, those monarchs who from ignorance of this, desire to subjugate king Nila’s city, are consumed by Hutasana (Agni). And from that time, O perpetuator of the Kuru race, the girls of the city of Mahishmati became rather unacceptable to others (as wives). And Agni by his boon granted them sexual liberty, so that the women of that town always roam about at will, each unbound to a particular husband. And, O bull of the Bharata race, from that time the monarchs (of other countries) forsake this city for fear of Agni.