/spoiler
After completing "Unified Front" & A Family's Legacy, you can then start the quest "Myths and Minotaur's".
After completing this quest a series of quests will then be unlocked and must be completed In order to 'Defeat the Minotaur' (Recollections, Full Circle, Seeking Answers & He Waits"
After these steps have been completed you can go back to the starting location of the quest and begin the boss fight!
(The starting location is in 'Minos' Legacy in Messara, an Island In the bottom right corner of map.)