This chapter of our guide for Assassin's Creed: Odyssey contains descriptions of all Ainigmata Ostraka that can be found on Kythera Island.
There is only one Ainigmata Ostraka on Kythera. This one is called Made in Abyss.
Made in Abyss
The location of this Ainigmata Ostraka: Go to the Temple of Aphrodite. The Ainigmata Ostraka is on the right side of the temple.
Puzzle solution: Ogylos Cave is located south of Skandeia Bay (Chytra Atoll). Go inside - the item is in the water.