This chapter of our guide for Assassin's Creed: Odyssey contains descriptions of all Ainigmata Ostraka that can be found on Abantis Islands archipelago - Skiros (Beneath Theseus's Dreams) and Eubea (A Deal for Freedom) are part of it.
There is only one Ainigmata Ostraka on Eubea. This one is called A Deal for Freedom.
A Deal for Freedom
The location of this Ainigmata Ostraka: Head to Oewos Quarry. The item is on the table.
Puzzle solution: Head to the northern part of Eubea. The item is on the altar right next to the bull's monument.
There is only one Ainigmata Ostraka on Sikiros. This one is called Beneath Theseus's Dreams.
Beneath Theseus's Dreams
The location of this Ainigmata Ostraka: Head to the Pirate Hideout. The item is on the table.
Puzzle solution: Go to the water body located in the southern part of the island. Dive to retrieve the item.