After finishing the original trilogy, I went back and wrote a prequel which takes place before the beginning of Taken Child. The prequel stands alone, and can be read either at the beginning or the end of the series.
Better to Burn. When Keanrih refuses to join with the wife his family has picked out for him, his father, Nocc mac Deira, hires Lythril to change his mind. You can't hire a dark user and expect all hell not to break loose; and Nocc soon discovers he's in over his head. While Theamh, Morat and the members of the Order's Lythril Commitee struggle to keep Keanrih out of her clutches, Lythril tells the story of what she hopes will be her ultimate triumph to an unidentified listener who may or may not be about to become one of the Dark One's most insidious agents.