Anpu was once the king of Duat, but after Azar's return from the dead, the other gods bid Anpu to step aside. After all, Azar now knew death in a way no one else did. Anpu did not agree with this claim, but he also stepped aside and went to dwell in Neter-Khertet, the border between life and death. He became the Opener of the Way and the ruler of that borderland.
Every mummy, upon entering a death cycle, will eventually find their way to Anpu. As the Opener of the Way, the Nameless Jackal leads the Arisen back to their body, or the tomb where their body will re-form. He sometimes also imparts some important lesson or piece of wisdom, though he rarely speaks.
Dreams of Avarice
The Heretic's pamphlet on Apotheosis frames Anpu as the son of Sutekh, the Nameless Serpent, who was himself a peer of the Judges of Duat. Sutekh demanded that the living should be judged for their deeds instead of being consigned at random to the jaws of Ammut, and created Anpu from his eye to advocate for them.
The text's description of the Rite of Return describes Anpu as the Guardian of the First Gate, who aided the would-be Arisen if they spoke his True Name. In the footnotes, the translator notes that Neter-Khertet is Twilight, and that Anpu is capable of interacting with any being in Twilight regardless of its "frequency."