The Diaries of Eurypheus are hidden collectibles found in the The Fate of Atlantis DLC. They're a quest item - but the actual quest has yet to be revealed (we expect maybe it will show up it in Episode 3).
(Click the link in the title to show exact location) Found next to a flower pot beside the Heart of Elysium viewpoint.
Inside the Collapsed Tower viewpoint building - on the floor just below the roof.
Found in Persephone's Akropolis - nearby the northern-most Downward Wings of Hermes.
Found at the southern-most tip of Minos's Faith - beside a picnic.
To follow the story - you'll need to find the Diaries of Orphedice hidden in The Underworld