You must find all 100 Richard's Flags in the Kingdom.
Note: For one of these flags you will need the Ledge Grab ability, which is received after completing Memory Block 3, so do not start looking for these flags until this ability is given back to you.
for an awesome guide to the Kingdom flags (as well as all other flags) and the Templars.
Please give full credit to FiReStOrM^
for his awesome work and dedication to making this incredible resource.
In case his page goes down again, I have left videos of all 100 Richard's Flags in the Kingdom, credit goes to youtube user TheGameStory