What are Ainigmata Ostraka?

Ainigmata Ostraka

These tablet writings unlock new engravings and are divided to two parts. The first part is the clue, which will give the location and description of the actual tablet’s location. The second part is finding the actual Engraving which is pinpointed by the tablet. The clue tablet is usually found in easily accessible areas while the actual engravings are located in more difficult locations such as deep caves and forts.

You can review the AOs you have in possession by accessing your Inventory then selecting Browse Bag in the lower left corner of the inventory screen. Completed AOs will have a check mark.

AOs can be spotted by Ikaros while flying and you'll know if there's one in a location since it will be part of the location objectives that you'll need to find to "complete" a location and get the bonus EXP.

There’s a total of 60 AOs scattered in the game. Finding the first AO will unlock its corresponding Engraving. Finding more AOs related to that engraving will increase the engraving’s tier and effectiveness. You’ll find the list of engravings and their corresponding AOs, arranged by region and recommended level where you’ll find them.

Take note that the tier of an Engraving will only increase by finding more AOs belonging to that engraving. You can’t just immediately skip the AOs found in the lower-level areas and hope to get a tier 5 engraving of that AO instantly - it doesn’t work that way. Find and recover them one step a time since there’s really no immediate rush in collecting all of them.

Engraving List

Reference: https://www.primagames.com/games/assassins-creed-odyssey/guides/assassins-creed-osyssey-eguide