You can find the document at the Epidauros Akropolis located in the similarly named city of Epidauros, and the tablet is located behind the akropolis on the Southwest side, surrounded by pillars at the face of a cliff, and usually guarded by a captain. Travel southeast along the edge of the Valley of Dreams.

Skylax the Fair is the first cultist of the Peloponnesian League, and is one of the few cultists to also be a region leader. In order to get the info on Harpalos needed to reveal his identity, you must search for cult-like enemies called the Followers of Ares - not Cultist Guards found at sites like the Snake Temple or Tomb of Tityos. There is something fishy going on amongst the mess of records, and that is where you will find me.

At the island, check around the East side of the island for a small fishing camp, where you'll find a spear sticking out of a basket that you can interact with to find the engraving. However, it seems that he can roam around the area, so make sure you reveal him before going on the hunt.