Son of Sāgra. Asamanja or Asamanjan was son of Sagara and Keshini (In some Purana's it is Sumati).
Sagara had 60,000 sons from wife Sumati and one son from Keshini.
According to Valmiki's account in Ramayana, he was a wild and wicked young man. He used to throw young boys playing in Sarayu river into great depths and see them drown.Thus his father Sagara exiled him. But his son Amshuman from his wife Ambujakshi succeeded Sagara as the king of Ayodhya.
According to Srimad Bhagavatam Chapter 8 of canto 9, In his previous birth, Asamanja had been a great mystic yogi, but by bad association he had fallen from his exalted position. Now, in this life, he was born in a royal family and was a jatismara; that is, he had the special advantage of being able to remember his past birth. Nonetheless, he wanted to display himself as a miscreant, and therefore he would do things that were abominable in the eyes of the public and unfavorable to his relatives. He would disturb the boys sporting in the River Sarayu by throwing them into the depths of the water. Because Asamanja engaged in such abominable activities, his father gave up affection for him and had him exiled. Then Asamanja exhibited his mystic power by reviving the boys and showing them to the King and their parents. After this, Asamanja left Ayodhya. All the inhabitants of Ayodhya saw that their boys had come back to life, they were astounded, and King Sagara greatly lamented the absence of his son.
According to the IIari-vansa, he was afterwards famous for valour under the name of Panchajanya.