Mercy's crest

You wear it while dealing with the cult on the coast, it makes you able to walk in without fighting everyone and challenge their leader and you can get them to join the inquisition (you have to wear it) or else you will fight all of them and no help from them.

Basically they are Andraste's warriors in a sense, so if you are playing the "I Am Andraste's Chosen" path then go for them, if not meh kill em if you want :P