Idunna is involved in the quest, Enemies Among Us.
Recently several templar recruits have gone missing. Tasked with finding the cause, Hawke finds that the missing templars frequented the Blooming Rose. Viveka, a book keeper at the brothel, informs Hawke that the templars in question were customers of Idunna. When Hawke finds Idunna, she uses Blood magic to force Hawke to tell of Viveka saying she was the last one with the templars and attempts to make Hawke slit their own throat. However, Hawke manages to overcome the spell. At this point, Hawke can either kill her or spare her and hand her over to the templars. Regardless of which is chosen, Hawke will find that she was linked to a plot to implant templar recruits with demons and return them to the order, sowing chaos within the order's ranks.
If Idunna is spared, she will send Hawke a letter revealing she repented and is now following the Maker, and that she wants evil tomes destroyed that people can learn blood magic from, but she can't because the templars will not permit her to leave her cell.
If you have any mage companions in you party at the time of the conversation with her, you have the option of asking them to interrupt Idunna's spell. Doing this later gives you a (+5) with that character. If Hawke is a mage, then they may simply break Idunna's control themselves, a warrior Hawke with the Templar specialization can also break free. However, there will be no friendship/rivalry points rewarded for doing so.
Killing her results in:
Sparing her and sending her to the templars: