Reading Guide. In order to obtain an heir, King Dasharatha conducts a horse sacrifice in which the chief wife, Kaushalya, plays a leading role. In the end, the gods promise that Dasharatha will have not just one son, but four!
Image: The image below shows Dasharatha's horse sacrifice; it's a 17th-century illustration. You can read more about the ancient horse sacrifice ritual at Wikipedia: Ashwamedha.
Source. Indian Myth and Legend by Donald A. Mackenzie (1913). [300 words]
At length Dasharatha resolved to perform the Ashwamedha [horse sacrifice] so that the gods might be prevailed upon to grant him an heir who would perpetuate his race. When his will was made known to the queens, their faces brightened as the lotus brightens at the promise of spring.
So it came to pass that a horse was let loose on the night of the full moon of the month of Choitro. A brahmin accompanied it and, after wandering for a full year, the animal returned again to the kingdom. Many rajahs attended the ceremony which took place on the north bank of the Sarayu river. Twenty-one sacrificial posts were set up for the birds, and beasts, and reptiles which were to be offered up besides the horse, and there were eighteen Homa pits.
When the fire was kindled upon the altar, Kaushalya, the chief queen, slew the horse with the sacred scimitar, while the brahmins chanted mantras. All night long Kaushalya and Kaikeyi, wives of the Maharajah, sat beside the horse's body, as was needful in performance of the rite. Portions of the flesh were duly given to the fire, and when the ceremony was completed, Dasharatha awarded great gifts of kine and treasure to the brahmins.
An oblation was afterwards offered to the gods, who came to the place of sacrifice with the music-loving Gandharvas, the celestial saints, the Siddhas, and seven Deva-rishis.
Brahma came with Vishnu and Shiva, and Indra came also with the hastening Maruts. Ere they departed, the gods promised that four sons would be born to Dasharatha.