Carlos Again's solution is spot on, however I just wanted to add that if you were looking for this achievement
Then you need to not only join this covenant, but also level it to acquire both spells.
FUN FACT: Once you do that, and get everything you need from her and Eingyi, then you can actually kill her for a Firekeeper's Soul. this will piss of Eingyi, but who cares! That guy is slow as hell