Use Eldritch Altars in each of the following ways.
When you slay monsters in maps that are influenced by The Searing Exarch and The Eater of Worlds, an altar may appear at their corpses. These Altars can be used to further control the risk and reward of these maps.
Eldritch Altar effects can be modified through the Atlas keystones Wrath of the Cosmos and Eldritch Gaze, making the modifiers more rewarding but also significantly more deadly.