Legacy of the First Blade is the last mission of the third part of Assassin's Creed Odyssey: Legacy of the First Blade.
Your first objective is to go to the Temple of Zeus.
There, you and Darius have to face the cultists' leader. The best tactic is to run away from the enemy and shoot him with fire arrows. Darius will focus his attention.
The last objective is to go to your son. This ends the mission and the DLC.