Idunna is a prostitute in the Kirkwall brothel, The Blooming Rose, as well as a Blood Mage.
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 blood 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 2 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.
If you are playing a mage then you are capable of breaking the spell and conversing with her, but there will be no friendship/rivalry points rewarded for doing so.
Idunna and male Hawke in The Blooming Rose
Killing her results in:
Isabela: friendship (+5)
Carver: friendship (+5)
Sparing her and sending her to the templars:
Isabela: rivalry (+5)
Anders: rivalry (+5)