
Once, as the account goes, under the guidance of Lord Vishnu, the Devas and Danavas were churning the ocean, i.e., doing Samudra Manthan in order to obtain Amrita, i.e., nectar. As soon as the deadly poison named Halahala Visha emerged from the churning, chaos spread across all the worlds. Due to the effect of the poison, all living beings began to experience difficulty in breathing. Seeing the entire creation on the verge of destruction, Lord Shiva drank the Halahala poison to protect the universe. Lord Shiva held that harmful poison in His throat, due to which His throat turned blue, and He became famous as Nilkantha.
Due to the effect of the Halahala poison, the body of Lord Shiva began to burn intensely, and He started experiencing extreme heat. Even after trying various remedies, when Shiva Ji's pain did not subside, Chandra Deva appeared there and said - "O Bholenath! I am renowned in creation for my coolness. Therefore, kindly grant me the fortune of being placed upon Your head. By doing so, the heat generated by the poison will certainly be pacified, and You will experience coolness."
Accepting the request of Chandra Deva, Lord Shiva placed him upon His head, due to which He attained great coolness and the pain caused by the poison came to an end. At that time, being pleased, Lord Shiva granted Chandra Deva a boon and said - "O Chandra Deva! You have relieved Our suffering with your coolness. We grant you this boon that from today onward, in this world, on Mondays, along with Our worship, your worship will also be performed. Whoever observes a Vrat dedicated to you with full devotion and offers Arghya in the evening to complete the Vrat, all their mental troubles shall come to an end, and they shall attain peace and happiness." Thus, by the blessings of Lord Shiva, Chandra Deva became worshipped in all three worlds.
According to the legend found in the Agni Purana, Lord Brahma created the Manasa Putras for the purpose of creation. Among these Manasa Putras was Sage Atri. Sage Atri was married to Devi Anasuya, the daughter of Maharshi Kardama. Devi Anasuya received three sons as a boon from the Trideva, who became famous as Sage Durvasa, Lord Dattatreya, and Soma, that is, Chandra Deva.
In time, Daksha Prajapati married his 27 daughters to Chandra Deva. After marriage, Chandra became deeply attached only to Rohini and began neglecting his other wives. Hurt by his behavior, the other daughters of Daksha expressed their sorrow before their father. Daksha tried to explain this matter to Chandra Deva in many ways, but being deeply attached to Rohini, Chandra Deva paid no heed to his words. Ultimately, Daksha became enraged and cursed Chandra Deva, saying - "O Chandra Deva! The beauty and charm in which you take pride and due to which you are insulting your other wives, that very form of yours will be afflicted with Kshaya Roga, i.e., Tuberculosis."
Due to the curse given by Daksha Prajapati, Chandra Deva was immediately afflicted with a wasting disease. As soon as Chandra Deva became afflicted, the flow of coolness and nectar-like essence on Earth was disrupted. Distress spread everywhere. Chandra Deva was deeply distressed and worried. He realized his mistake and expressed his suffering to Indra, the king of the Devas. Upon Chandra's request, Indra and other Devas, along with sages like Vashistha, approached Lord Brahma for a solution. Brahma Ji said - "O Chandra! For relief from this curse, you should worship Mrityunjaya Lord Shiva at the sacred Prabhasa Kshetra along with the other Devas. By His grace, you will certainly be freed from the disease."
Following the instructions of Brahma Ji, Chandra Deva performed severe penance using the Mrityunjaya Mantra of Lord Shiva. Pleased by his penance, Lord Shiva in His Mrityunjaya form appeared before him and asked him to seek a boon. Chandra requested freedom from his disease. Lord Shiva granted him the boon of immortality and said - "O Chandra Deva! Don't be distressed. By the power of My boon, your curse shall be mitigated." Saying this, Lord Shiva placed Chandra upon His head and became established as Someshvaranatha.
Lord Shiva said - "While maintaining the dignity of Daksha Prajapati's curse, We will not completely nullify it. However, during the Krishna Paksha, one of your Kala, i.e., phases shall diminish each day, and during the Shukla Paksha, it shall gradually increase again. Thus, on every Purnima, you shall regain your full form. Furthermore, whoever observes a Somavara Vrat and Puja dedicated to you and offers Arghya will be freed from various diseases." Upon receiving this boon, all beings in the world became delighted. Chandra Deva once again resumed his function of spreading nectar-like coolness in all directions as before.
॥Iti Shri Chandra Somavara Vrat Katha Sampurnah॥