SB 5.26.6
yatra ha vava bhagavan pitr-rajo vaivasvatah sva-visayam prapitesu sva-purusair jantusu samparetesu yatha-karmavadyam dosam evanullanghita-bhagavac-chasanah sagano damam dharayati.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The King of the pitas is Yamaraja, the very powerful son of the sun-god. He resides in Pitrloka with his personal assistants and, while abiding by the rules and regulations set down by the Supreme Lord, has his agents, the Yamadutas, bring all the sinful men to him immediately upon their death. After bringing them within his jurisdiction, he properly judges them according to their specific sinful activities and sends them to one of the many hellish planets for suitable punishments.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Yamaraja is not a fictitious or mythological character; he has his own abode, Pitrloka, of which he is king. Agnostics may not believe in hell, but Sukadeva Gosvami affirms the existence of the Naraka planets, which lie between the Garbhodaka Ocean and Patalaloka. Yamaraja is appointed by the Supreme Personality of Godhead to see that the human beings do not violate His rules and regulations. As confirmed in Bhagavad-gita (4.17):
karmano hy api boddhavyam
boddhavyam ca vikarmanah
akarmanas ca boddhavyam
gahana karmano gatih
“The intricacies of action are very hard to understand. Therefore one should know properly what action is, what forbidden action is, and what inaction is.” One should understand the nature of karma, vikarma and akarma, and one must act accordingly. This is the law of the Supreme Personality of Godhead. The conditioned souls, who have come to this material world for sense gratification, are allowed to enjoy their senses under certain regulative principles. If they violate these regulations, they are judged and punished by Yamaraja. He brings them to the hellish planets and properly chastises them to bring them back to Krsna consciousness. By the influence of maya, however, the conditioned souls remain infatuated with the mode of ignorance. Thus in spite of repeated punishment by Yamaraja, they do not come to their senses, but continue to live within the material condition, committing sinful activities again and again.