The Prosecutor is a traveller from Sarpeidon's present hiding in an era of witch trials in All Our Yesterdays, who examines the imprisoned Kirk and is forced to guide him back to the library.
The Prosecutor is a native of Sarpeidon in the Original Series episode All Our Yesterdays. Like the rest of his people he has escaped the coming nova of his sun by passing through the atavachron into the planet's past, in his case an age resembling seventeenth-century England.
Kirk blunders into the same period, is heard talking to disembodied voices and is jailed as a witch. The Prosecutor, a man of rank, comes to question him and gives himself away when he recognises talk of the library and Mr. Atoz.
He is terrified that any link with the stranger will send him to the stake as well. Kirk threatens to denounce him as a fellow witch unless he helps, and the Prosecutor leads him back to the spot in the alley wall where the portal lies. He himself has been prepared by the atavachron and cannot return.