I'm basically a paladin, no matter what I'm actually playing in the game, so of course I freed/saved the Nightsong (who is AWESOME). Unfortunately that meant killing Shadowheart, but I wasn't really surprised - I'd mostly ignored her up until that point.
Did you have a low relationship with Shadow Heart?
At high relationships, she drops the Nightspear, saves Nightsong, and then has the option to turn her faith to Selűne
Such options exist
