This mission begins with meeting Herodotos at the Thermopylai. It is in Malis near the ocean on the eastern side. Look for a number of columns and a giant statue of a lion.

Upon arrival, a cutscene with Herodotos will play. He explains the history of the Thermopylai and the significance of the spear. A flashback to Leonidas will play. Herodotos will then explain he knows of an ancient civilization and strange shapes on the islands of Andros.

Sail to Andros; it's in the southwestern part of the map. Once at the island, call Ikaros. follow the path until coming to the large door that looks very uncharacteristic for ancient Greece.

Interact with the door. A flashback will play and the spear will open the door. Follow the path down to a structure that has a space for a spear. Interact with it. This is the first of the upgrades for the spear in Upgrade the Spear, and the place you will go every time you've collected enough fragments to upgrade it again.

The screen will go black and the game will return to the present day. After a short cutscene and conversation with Victoria, take control of Layla. This particular part of the game isn't incredibly important but does allow for some time to catch up on the situation and context about what's happening on Layla's end.

Read Layla's emails, run around the room and find some Easter eggs and references. When done, interact with the animus to return to Greece.

Upon returning to Greece, another cutscene will play with the player's character confronting their sibling. If playing as Kassandra, this will be Alexios, and if playing Alexios, it will be Kassandra. Choose to fight him her or not.

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When things with Deimos has wrapped up, return to Herodotos on the Adestria. After a short conversation with him, the mission will be complete.