Astrologists are encountered in the Crown of the Old Iron King DLC. They can be found through the Iron Passage.
When engaged in combat, Astrologists will often open by casting Promised Walk of Peace or Profound Still. If given enough time, they will attempt to cast both, severely limiting the player's options for fighting them.
After applying one, or both, Hexes, the Astrologists will use area of effect Miracles, such as Heavenly Thunder and Blinding Bolt, to deal damage. They may also cast Soothing Sunlight to heal themselves and nearby enemies.
The Sanctum Priestess Tiara can be used to nullify the effect of Profound Still, allowing casters more freedom when fighting Astrologists.
Black Hollow Mage Hood