What Is Jaqen H'ghar's Mission In Westeros?

Jaqen H'ghar helps Arya Stark escape Harrenhal and when he leaves he tells her he has duties, and that he has promises to keep. We later see him at the citadel in old town getting a key that opens every door in the citadel. So with everything we know about Jaqen and where he has been seen, what is are these duties and promises? What is Jaqen H'ghar's mission in westeros? Has he gone rogue or is her serving the faceless men in some larger mission?