Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 21


यक्षरक्षांसि भूतानि पिशाचा: पिशिताशना: ।भूतेशवत्सा दुदुहु: कपाले क्षतजासवम् ॥ २१ ॥


yakṣa-rakṣāṁsi bhūtānipiśācāḥ piśitāśanāḥbhūteśa-vatsā duduhuḥkapāle kṣatajāsavam


Then the Yakṣas, Rākṣasas, ghosts and witches, who are habituated to eating flesh, transformed Lord Śiva’s incarnation Rudra [Bhūtanātha] into a calf and milked out beverages made of blood and put them in a pot made of skulls.


There are some types of living entities in the form of human beings whose living conditions and eatables are most abominable. Generally they eat flesh and fermented blood, which is mentioned in this verse as kṣatajāsavam. The leaders of such degraded men known as Yakṣas, Rākṣasas, bhūtas and piśācas are all in the mode of ignorance. They have been placed under the control of Rudra. Rudra is the incarnation of Lord Śiva and is in charge of the mode of ignorance in material nature. Another name of Lord Śiva is Bhūtanātha, meaning “master of ghosts.” Rudra was born from between Brahmā’s eyes when Brahmā was very angry at the four Kumāras.