The third mission in Assassin's Creed: Odyssey Fate of Atlantis consists of several tasks: Next Lesson, The Underworld's Fallen Guardians, Beyond the Veil and Pay the Ferryman
Pay the Ferryman
Your objective is to talk to Charon. You will find him in the middle part of the map - he will move toward you in a boat. Speak to him to unlock two missions - Beyond the Veil and A New Lease on Death.
Beyond the Veil
The goal of this mission is to obtain a special armor. You will get it by killing Poisoner, Epiktetos, Deianeira and Swordfish. Exact information about this armor is on a separate page of the guide.
This mission is based on finding 3 Keeper's Insights. All of them are described in a separate section of the guide.
The Underworld's Fallen Guardians
You complete this mission by killing Herakles, Achilles, Perseus and Agamemnon. These opponents are highlighted on the map - their symbol is presented in the screenshot.
Labors of the Keeper
This mission completes this part of the DLC. To complete this mission, you must defeat Hades. The detailed description of the fight against Hades can be found on a separate page of the guide.