The Cardinal has come to the conclusion that there is a Templar spy within the Inquisition. You would be his first suspect if you'd known about the high inquisitor's visit - fortunately you didn't (or so he thinks), so he wants to use you to find the infiltrator.
There is no fighting required for this mission, just the usual back and forth between Santiago and Tortuga. Talk to Mogens Brisebois in Tortuga, then return to Santiago to enter the separate instance for this mission. You have to search the room - there will be some papers on the writing desk next to the bed. Click on them, then return to the Cardinal for the reward and the next mission.
- Find Evidence