Parashuram is the sixth incarnation of Bhagavan Srihari Vishnu. It is believed that he went around the world 21 times and killed Kshatriya kings who had moved away from Dharma. As per Brahmanda Purana, Kartavirya Arjuna forcefully seized the wish fulfilling cow Kamadhenu from the hermitage of Sage Jamadagni.
Sage Jamadagni protested this unruly behaviour of a king. Kartavirya Arjuna beheaded the sage and took away the cow.
Jamadagni’s son, Parashuram, heard about his heinous incident from his mother Renuka Devi.
Whiling narrating her husband’s death, Renuka Devi struck on her heart with her hands 21 times in sorrow.
Parashuram who witnessed her wailing took the solemn vow that he would go around the world 21 times and wipe out all the kings who practice Adharma.
This is mentioned in the Brahmanda Purana.