Assassin's Creed Odyssey

To unlock this achievement you need to find and kill the Minotaur. 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.

This is achieved by travelling to the island of Messara at the very bottom of the map and going to the Minos's Legacy area and finding a side quest called 'Myths and Minotaurs' given to you by Ardos. Once you have completed this short introductory quest you will be given a new main quest called 'Of Minotaurs and Men' that is split into three sub-quests. These sub-quests are each quite long with multiple steps to follow, but they are all linear and easy to complete. After completing these three quests, return to Ardos for the next phase of the questline called 'He Waits'.

Note that the level recommendation to start this questline is level 33, but the final quest 'He Waits' is recommended level 40, so you can return to it later or do the quests in smaller chunks. Also you can do this quest before ever getting the prompt from Pythagoras in Atlantis, unlike the other three Legendary creatures.