In order to unlock this Trophy you must carry a grimoire to the end of the level. The grimoire takes up your potion item slot and it also lowers the health pool of each member of your party by 30%, which can make the missions a bit more tricky. As a result I recommend doing this one Easy. The video below by Elgato Gaming and posted by 360GameTV (I take no credit for this video) does an excellent job of showing how to find both of the grimoires in the prologue mission, The Horn of Magnus, (but you only need one tome for this Trophy)...enjoy!