Long long ago, in a place moderately far away - Korvosa, a port city that broke away from Cheliax during the civil war and has maintained its independence (after a fashion (if you don't count the fact that a Duke of Hell is secretly in power in the Academae)) - a little girl was born.
Her mother was the archmage Myrabelle, one of the most powerful figures in the Inner Sea, a Lawful Evil archmage whose name was rarely spoken in voices not heavy with dread, the one woman Terthune Thrune never dared to cross. Her father was a Chelish count who was quite surprised, though not wholly displeased, to learn that Myrabelle had decided to have him as a lover for precisely as long as it took her to conceive a child and not longer. He met Lilia every few years, when business took him to Korvosa.
She had a sister, by a different father. Rose. Rose was older; Lilia was stronger. They were vicious to each other; Lilia was better at it. Their mother punished disobedience, destructiveness, but not cruelty. She knew what she was raising them for.
She would need them to conceal their surface thoughts, to be able to comprehensively fool mindreading. They would need to be able to lie under a truth spell. There was a known song-sorcerer spell that could enable that. That wouldn't be good enough, of course, but it was a starting point. She got them wands of it, at unfathomable cost. She taught them to use them. She made them lie to her.
She tortured them. They would need to be people who could not break under torture.
They loved her. They were mind controlled to love her, and also she was their mother, and that might have been enough all by itself. She was not the kind of person to find out.
The thing that was being asked of them was impossible. Rose, eventually, gratefully accepted the opportunity to serve her mother some other way, by bearing children Myrabelle needed for a dark experiment.
Lilia learned not to think any thought she did not wish to think, and then to think them silently and distantly where no spell could reach them. Lilia learned how a truth spell gripped your mind and how to have nothing in your mind for it to grip.
Lilia sold her soul to Hell. To a devil under Mephistopheles, particularly. Her mother had compacted for controlling rights. Lilia went to work in Egorian.