Bhagavatapurana 9.9.25

9.9.25

rākṣasaṃ bhāvamāpannaḥ pāde kalmāṣatāṃ gataḥ /
vyavāyakāle dadṛśe vanaukodampatī dvijau //

25. He became a Rakshasa and had his feet turned black (as if scorched by fire); he once saw a brahmin couple living in the forest in the act of their intercourse.