The story of Vṛṣeśa (Vṛṣa, Vṛṣabha, the incarnation of Shiva in the form of a bull) is told in chapters 22-23 of the 'Śatarudra-Samhita' of 'Śiva-Mahāpurāṇa'.
When the devas and asuras, united together, churned the milky ocean, many unique and wonderful things rose to its surface. During churning the ocean, from the sprays and drops of nectar (amṛta) that appeared when it came to the surface, many damsels of amazing beauty came up. They were taken by the asuras for their own enjoyment. Having defeated the Daityas in battle and chased them even to Pātāla, Viṣṇu saw these beautiful damsels. Hi grew amorous of them and thus thousands of sons were born by them.
Soon, these sons of Viṣṇu, born in Pātāla, started to harm the dwellers of the earth and heaven. Then, in order to rectify this situation, Lord Śiva took the form of a bull (Vṛṣa) and descended in this guise through a crevice in the ground to Pātāla. The sons of Viṣṇu attaked him, but were killed by him in the ensuing battle. After that, Vṛṣeśa was attacked by Viṣṇu himself, who, being deluded by māyā of Lord Śiva, could not recognize Him. In the ensuing fighting, Vṛṣabha defeated Viṣṇu. When his pride fell off, Viṣṇu recognized Lord Śiva, who, after explaining to him his mistakes, ordered him to leave Pātāla.
Then Viṣṇu advised all the gods go to Pātāla to have sexual pleasures with many damsels born from drops of nectar. To prevent this, Śiva said that anyone who enters Pātāla, excepted of the sages and Daityas, born of His parts, will die immediate. From that moment on, the regions of Pātāla became a inaccessible place for people.