Chapter 40 of 59 · Rudra Samhita - Yuddha Khanda · Overall chapter 210 of 464
Shankhachuda Is Killed
With his army destroyed, Shankhachuda demands single combat; a voice from the sky reveals that he remains unkillable only as long as he wears Krishna's kavacha and his wife Tulasi stays faithful, so Shiva sends Vishnu to steal both at once - disguised as a brahmana to take the kavacha, then as Shankhachuda himself to violate Tulasi - clearing the way for Shiva's trident to finally end him.
Sanatkumara said
The danava saw that large parts of his army were destroyed, and that the brave ones he loved as his own life had been killed. He grew extremely angry and spoke to Shambhu, telling him to stand firm and asking why his men were being killed when he himself stood in front of him, so that Shiva should fight him. Having said this, the danava lord set himself for the clash and, determined, advanced on Shankara. He hurled divine weapons at great Rudra and rained arrows like clouds pouring rain. He used many kinds of illusion and showed many terrible things, but the excellent devas and ganas did not believe any of it. Shankara playfully released the maheshvara weapon, great and divine, which destroyed every illusion. The energy of the maya was quickly destroyed, and the divine weapons lost their force. To kill the danava in the battle, the very strong Mahesha suddenly took up his trident, which was extremely energetic and could not be countered.
An unseen voice from the sky stopped him. It asked Shankara to hear a prayer and not hurl the trident yet. In every way, it said, he could destroy the universe in an instant, so there was no doubt he could deal with one danava, but he should not break the ordinances of the Vedas. It asked him to hear and make true a certain condition. As long as Shankhachuda wore Hari's powerful kavacha on his arm and as long as his wife stayed faithful to her husband, he would suffer neither age nor death. These were Brahma's ordained words, and they should not be made false. Hara heard the voice and agreed.
Following Hara's wishes and instructions, Vishnu, who knows maya best, went there in the disguise of an aged brahmana. He approached Shankhachuda and said that he had come to ask for alms, but would not say what until the danava had given his promise, since he was kind to the needy. Delighted in face and eyes, the king agreed. The old man then deceived him, saying: "I want the kavacha." The danava lord loved brahmanas and kept his word, so he gave the divine kavacha, dear to him as his own breath, to the brahmana. In this way Hari took the kavacha by maya. Then he took on Shankhachuda's form and went to Tulasi, and so accomplished the gods' task.
Shambhu thus kept his word, and to kill Shankhachuda he took up his blazing trident. That trident of Shankara, the supreme self, is named Vijaya. It blazed in the directions and lit up the space between heaven and earth, like a crore midday suns or the fire of dissolution, impossible to counter and unconquerable, infallible, the destroyer of enemies. A fierce circle of energy surrounded it, and all the shastras, astras and arrows were within it. Gods and asuras could not bear it. When he wishes to destroy the whole universe in his play, all energy gathers in that blazing supreme weapon. It was a thousand bow-lengths long and a hundred hand-lengths wide, the form of the individual self and of brahman, eternal in form and never made. For a while the trident spun in the sky above Shankhachuda's head. At Shiva's command it swooped down fast and reduced him to ashes. Having done its work, Mahesha's trident sped back through the sky at the speed of thought to Shankara.
Drums sounded in heaven, gandharvas and kinnaras sang, sages and devas offered praise, and apsaras danced. Flowers fell on Shiva from every direction, and Hari, Brahma, Shakra and the others, with the sages, praised him. Through Shiva's compassion Shankhachuda was freed from his curse and regained his former form. Conch shells came out of his bones. Water from a conch is pleasing to everyone except Shankara; Hari and Lakshmi and those connected with them especially love it, but, Sanatkumara said to the sage, this is not so for Hara. Having killed him, Shankara returned happily on his bull to Shiva's world, with Uma, Skanda and the ganas. Hari went back to Vaikuntha and Krishna became well again. The gods, their realms regained, were full of joy, every obstacle was removed, the world returned to normal, the sky was clear and the earth auspicious in every way.
Sanatkumara closed by saying that this account of Mahesha's deeds brings happiness, removes misery, grants prosperity and satisfies every desire, bestows fame and long life, counters obstacles, and gives enjoyment and liberation as fruits. A wise person who always hears about the moon-crested lord, makes others hear it, reads it and has it read will without doubt gain wealth, grain, sons and happiness and every wish, and in particular will become devoted to Shiva. The account is unmatched, destroys calamity, produces supreme knowledge and increases devotion to Shiva. A brahmana who hears it gains the radiance of brahman, a kshatriya wins victory, a vaishya gains excellent wealth, and a shudra who listens becomes an excellent person.
Retold in our own words from Bibek Debroy's English translation of the Shiva Purana (Penguin Classics). Names, numbers and the order of events follow the source.