Idunna is a prostitute in the Kirkwall brothel, The Blooming Rose.
Involvement
During the quest, Enemies Among Us, 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 attempts to use magic to force Hawke to slit his/her throat. However, Hawke manages to overcome the spell. At this point, Hawke can either kill her or spare her and send her to the Templars. Regardless of which is chosen, Hawke will find that she was linked to a plot to implant Templars with demons and return them to the order, sowing chaos.
If Idunna is spared, she will send Hawke a letter in Act III in which she claims she is now following the Maker, though the Templars will not permit her to leave her cell.
If you have any mage companion 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 friendship score with that character. If Hawke is a mage, then he/she may simply break Idunna's control themselves.
Killing her results in:
Isabela: friendship (+5)
And forfeiting the Forbidden Knowledge quest.
Sparing her and sending her to the templars:
Isabela: rivalry (+5)
Anders: rivalry (+5)