Assassin's Creed Odyssey

At the beginning of the DLC you will be introduced to a new threat in the form of Tartaros Rifts and also be shown how to close them. You essentially have to enter the rift and defeat the 3 or 4 enemies that are created from it and then the rift will be closed. At first, you will find yourself weakened against the enemies that spill from them and later find out that acquiring the 'Armor of the Fallen' will aid you in your attacks against the rifts. You will need to acquire this armor as part of the main story so you should leave these rifts until you have acquired the armor. 4 pieces of the armor are acquired from defeating 4 of the 'Fallen', see "The One" for more details on these, while the fifth and final piece is found during a story mission towards the beginning of the story.

After acquiring the armor you can make your way through the locations in the game where Tartaros Rifts are present. Though you will find some as you're playing naturally, most are in generic locations that you would otherwise not visit during the story. All of their locations can be found on the maps below:

The Scorched Lands

Mourning Fields

Chasm of Torment