The following chapter of the Assassin's Creed Odyssey game guide describes the only Ainigmata Ostraka in the Obsidian Islands. This archipelago includes the islands of Melos and Hydrea. However, the only Ostraka in this area can be found in the northern part of Melos.
Upon Typhon's Crown
Below we have described how to find the Ainigmata Ostraka in Assassin's Creed Odyssey which is located in Melos, an island that is part of the Obsidian Islands. The Ainigmata Ostraka is called Upon Typhon's Crown.
Location of Ainigmata Ostraka: Inside the Leader House, in the northern part of Malos.
Ainigmata Ostraka riddle solution: Go to Adrestia. Stand on the rock by lava. This is where you will find the helmet.