Memory Fragments, 16 in total, are lore items that can be found in Radjen's Mind Palace, a new region added to the game in the Mask of Darkness DLC. Some are automatically collected when you reach certain locations, while others are obtained by breaking blue crystals scattered throughout the map. These fragments reconstruct Radjen's memories, revealing her past, motivations, history, etc.
To mark unfound Memory Fragment locations on the map, you can purchase a treasure map from Fariba in The Haven after completing the DLC.
Once you find all memory fragments, you will unlock the "Total Recall" achievement / trophy.
