The Sadhyas and Pitas, the denizens of Pitrloka, are invisible forms of departed souls, and they are created by Lord Brahma.
"The Pitas themselves took possession of the invisible body, the source of their existence. It is through the medium of this invisible body that those well versed in the rituals offer oblations to the Sadhyas and Pitas [in the form of their departed ancestors] on the occasion of sraddha."
"Those who are addicted to the worship of the demigods go to the planets of the demigods, and those who are addicted to worship of the Pitas, forefathers, go to the Pitas. Those who are inclined to material activities remain in the material sphere."
"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."
Bhaktivedanta Book Trust. Excerpted from various texts and purports of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.