Red-handed...

Talk to Amayiro to get your quest, it suggests that you find a map of Xarthan's house in the Order's Tower, but I couldn't find it. It says that Xarthan's house is in the Baker's District, so I started searching and found it at [-26,-50] near a paddock.

You'll need a Handyman to stand on the stairs to Xarthan's house and you can walk right in by standing on the same tile. Go to the second floor and grab a document from his desk. Head back to the Order's Tower at [-30,-53] and head up to the top floor, there you'll find Xarthan and you'll have the option to Threaten or Take his word for it... I threatened him since he sent me all over Brakmar. Return to Amayiro when you've dealt with Xarthan.