Amāvasu(Amavasu) is the son born of the womb of Urvaśī (Urvasi), and sired by Pururavā (Pururava)belonging to Candravaṃśa. However, in another śloka (sloka;verse) of Vāyupurāṇa (Vayupurana), he is referred to as the eldest son of Āyu (Ayu), son of Pururavā.
According to Purāṇa (Purana; Mythological Scripture), Acchodā (Acchoda), the daughter of Agniṣvāttva (Agnishwattwa) Pitṛgaṇa(Pitrigana) got attracted to Rājarṣi(Rajarshi; A King with the attribute of a sage) Amāvasu, and so she fell from Pitṛloka. In Mahābhārata (Mahabharata), King Uparicara(Uparichara) Vasu is mentioned in this context, instead of Amāvasu. King Amāvasu got a son called Bhīma (Bhima).