Arvavasu Takes the Vow for Paravasu's Sin
Paravasu returns to the sacrifice and confesses to his brother Arvavasu that he has killed their father, mistaking him for an animal in the dark. He tells Arvavasu that he alone cannot complete the expiatory rite — Arvavasu must perform the penance for killing a brahmana while Paravasu finishes the king's sacrifice.
The sacrifice of King Brihaddyumna was underway, conducted by the two brothers Paravasu and Arvavasu — both learned sages, both appointed as priests. But something had gone terribly wrong.
Paravasu had gone into the forest at night. In the darkness, he saw a shape moving and mistook it for an animal. He killed it. When he approached the body, he found he had killed his own father — the sage Raibhya.
He returned to the place where the sacrifice was being held and told his brother.
"You will never be able to accomplish this deed alone," Paravasu said. "I have killed our father, mistaking him to be an animal. Therefore, you observe the righteous rite that must be observed for killing a brahmana. I am capable of accomplishing the deed alone."
The sin of killing a brahmana — brahmahatya — was among the gravest in the world. It required a specific vow of expiation, a period of penance that would cleanse the killer. Paravasu was asking his brother to carry that burden for him.
Arvavasu did not argue. He did not ask why Paravasu could not do it himself. He simply accepted.
"Then you complete the intelligent Brihaddyumna's sacrifice," Arvavasu replied. "For your sake, I will control my senses and act so as to expunge the sin of killing a brahmana."
He left the sacrifice. He undertook the vow — the fasting, the isolation, the rituals of purification prescribed for one who had killed a learned brahmana. He performed it fully, faithfully, for his brother's sake.
When the vow was complete, he returned to the sacrifice. Aranyaka Parva, Chapter 436