Assassin's Creed Odyssey

To unlock this achievement you need to defeat Medusa. You acquire the quest to do this after first discovering Atlantis and speaking to Pythagoras who will charge you with reclaiming four keys to unlock Atlantis. The four keys are each held by one of the four Legendary creatures that you fight: The Sphinx, The Minotaur, Medusa, and The Cyclops. See "Birthright" for details on how to find Atlantis to start these quests.

To find Medusa you're going to want to travel to the island of Lesbos in the top right of the map. The structure, 'Petrified Temple' is roughly in the center of the clouded area on the western half of the island. To begin the series of quests that allow you entrance into this structure, you must pick up the initial quest 'Romancing the Stone Garden' found just south of this temple. Note this quest is recommended level 46.

After completing 'Romancing the Stone Garden', you will have to do the quest 'Love's Long Shadow'. This in turn leads to 'A Slithery Plea' which has two sub-quests: 'Hard to Artemis' and 'Heavy is the Spear'. Complete these to acquire the quest 'Keys to Happiness?' which in turn leads to 'Writhing Dead' that grants you access to the structure to fight Medusa. This quest was a level 50 quest so although you can begin the quest line earlier, the fight against Medusa will be extremely difficult if you attempt it too early. Defeat her in what has got to be one of the most memorable fights in the series and the achievement will unlock.