Harivamsa Purana and Mahabharata are complementary to each other.
Harivamsa especially describes the pastimes of the Supreme Lord that took place after the battle at Kuruksetra, including His pastimes of disappearance. Harivamsa is considered a supplement of Mahabharata.
Harivamsa consists of about sixteen thousand verses, it was first recited by the great sage, Vaisampayana, to King Janamejaya. In that assembly, Ugrasrava or Sauti was also present. Later on, by the request of the sages at Naimisaranya, headed by Saunaka, Sauti again recited the Harivamsa Purana.