With Aletheia's guidance, Alexios sought the three symbols that would unlock the Great Seal to Atlantis for the Heir of Memories, though he knew not who that was.
Three Symbols Entombed
Objectives of Three Symbols Entombed
1. Find the three symbols
You have to complete three support quests: Agamemnon's Symbol, Eteokles's Symbol and Orion's Symbol.
2. -
After collecting the last of the symbols, Layla will play for a while as Deimos and will torture Phidias to get the correct names of the symbols from him.
When you return to Atlantis, all you have to do is walk up to the door and use the appropriate symbols: Itira, Korgath, Metin.
3. Quest Completed
Rewards: Epic XP, Epic Drachmae