Updated: Dec 9, 2022
Parashurama was the sixth avatar of Vishnu from the Dashavatara.
Once upon a time, a king called Kartavirya Arjuna (not to be confused with the Arjuna of the Pandavas), forcibly took the sacred cow Kamadhenu which belonged to Parashurama's father and also killed him in the process, when Parashurama was away from the ashram.
When he found out what had happened, an angry Parashurama stormed the king's palace to kill him and get the cow back. He did not stop there though - he even killed all of Kartavirya Arjuna's son's, cousins and other relatives - the 'kshatriyas' who according to him had lost all their morals, and which was the reason that this avatar of Vishnu was prophecised to take birth in the first place. But even after this, his anger did not subside and he went away to do penance.