I have read about karmic debtors previously in materials related to the practice of Chod.
In the chod tradition, the four recipients of the offering thatone keeps in mind are called “four guests.” One imagines organising a big feast and inviting the four kinds of guests. They are:
1) Higher guests, Buddhas, Bodhisattvas, Yidams (deities), dakinis, sages, teachers, yogis and yoginis
2) Guests of Quality—fierce beings, wrathful protectors of the dharma, pure emanations of enlightened mind, and powerful worldly spirits
3) Guests of Compassion—existing in the six realms, humans, animals, hungry ghosts, hell beings, demy-gods and gods
4) Lower Guests—all beings to whom we owe a karmic debt, demonic forces, harm-givers, negative spirits and entities.
It is said that karmic debtors are those whose one is karmically linked with most strongly. One owes them something and they owe one something. One of the very strong karmic connections is there because, for some reason, one killed them and such a karmic connection is what is called “a negative karmic debt.” It is called a debt because the result of that connection has still not been resolved, and this negative/black karmic debt travels with and directs our minds into future rebirths.