Dasrath, although having everything and being so incredibly popular, loved and successful could not find any true happiness because he wanted a son, when he retired to dedicate his entirety of life to the service of the Supreme Lord, he could entrust the responsibilities of the kingdom to his son. Years passed and ultimately, he performed on the instruction of Vasishta muni a special yagna. The head priest was Rishya Sringa. The culmination of that yagna was the chanting of a mantra, an oblation. From that oblation, in the sacred fire, a divine personality emerged. He was holding a golden pot of celestial paysam or nectar. He gave that to Dasrath and told him to share it amongst his three wives- Kaushalya, Kaikayi and Sumitra. We know from Ramayan after three of them drank that paysam, they became pregnant. Ram, Laxman, Bharat and Satrugna were to be born.
