Send an accusation and the muse can only answer with “guilty” or “not guilty.” 

The Master’s legendary hubris falters.  Momentarily.

The accusation may refer to any number of people, but is likely to refer to Bill Potts.  She was his coup de grace of long-cons, a decade-long sting on the Doctor’s desire to be the salvation of all.  

And he is far less proud of that, in retrospect, than one might expect.  Long, painful conversations with the twelfth and thirteenth faces of the Doctor have resulted in something perilously akin to shame.

Because really, despite his nasty cracks at her “boring” personality, Koschei liked Bill. She was smart, easygoing, and far from petty in the way that humans ordinarily are.  He did, after all, learn all the mundanities of her personhood, in those ten years: her favorite toothbrush color and gum flavor.  The music she’d play while working out.  What gave her insomnia.  The name of the first girl she kissed.  

Which makes the whole betrayal ten times as horrifying.  He knows that.  He knows.  

        “Guilty.”