Richika arranged for a spectacular sacrifice. Some rice pudding was obtained from the yajna and Richika gave it to his wife Satyavati. He said, Split this rice pudding into two halves. Eat half yourself and give the remaining half to your mother. Here, let me divide it. This is your half and that is your mother's. We are brahmanas. So we will have a son who will display the traits of a brahmana. Your father is a kshatriya and your mother will have a son who will behave like a kshatriya.
Saying this, Richika went off to meditate in the forest. But mother and daughter managed to mix up their halves. In the course of his meditations, Richika realized that Satyavati was going to give birth to a brahmana son who would display kshatriya traits. Through his powers, he managed to postpone this birth by a generation. So Satyavati gave birth to Jamadagni. It was Jamadagni's son Parashurama who exhibited all the kshatriya like characteristics.
Gadhi's son was Vishvamitra. Vishvamitra was born a kshatriya. But because of the mixing up of the rice pudding, Vishvamitra turned out to be brahmana-like.
There was a king of the Haihaya dynasty named Arjuna. He had a thousand arms. He had also obtained the boon that flaming fire itself would be perpetually present on the tip of his arrow. Whenever he shot an arrow, the fire from the tip of the arrow burnt up the target. In this fashion, Arjuna used to burn up villages, cities and forests. He once burnt up the hermitages of sages. And one of the sages cursed Arjuna that he would killed by Parashurama.
Parashurama learnt the art of fighting from Shiva himself. While Parashurama was away learning how to fight, Arjuna arrived in Jamadagni's hermitage. Jamadagni had a wonderful cow (dhenu), known as a kamadhenu because it produced whatever objects one asked (Kama) from it. Using this kamadhenu, Jamadagni treated Arjuna and his entrie retinue to a royal feast.
Arjuna asked Jamadangi to give him this cow, but the sage refused. Arjuna then asked his soldiers to forcibly take away the cow. But just as this was going on, Parashurama arrived. He killed Arjuna, slicing off Arjuna's thousand arms in the process.
Having disposed of Arjuna, Parashurama went off to meditate and pay another visit to Shiva.
Taking advantage of Parashurama's absence, Arjuna's sons invaded Jamadagni's hermitage. They killed Jamadagni. When Parashurama returned, he exacted vengeance for this evil deed. He killed Arjuna's sons. Since Arjuna and his sons happened to be kshatriyas, Parashurama also killed all the kshatriyas in the world. He did this not once, but twenty-one times over. Why twenty-one times? The reason was that there were twenty-one weapon-marks on the dead Jamadagni's body.
But killing was a crime and Parashurama had committed a sin. As penance, Parashurama donated cows and performed a lot of tapasya. He also arranged an ashvamedha yajna. All this did not prove to be atonement enough. To complete the penance, Parashurama sought the advice of the sage Kashyapa.
Kashyapa told him to perform the donation that is known as tulapurusha. A tula (or tuladanda) is a pair of scales. The person (purusha) who is performing the donation is placed on one side of the weighing scale. On the other side are placed objects like honey , clarified butter, molasses, clothing and gold. The weight of the objects being donated has to be equal to the weight of the person performing the donation. This is known as tulapurusha. Parashurama performed tulapurusha and was freed from his sin.