You saved a cabal of mystics from being used by the Banished Pantheon.
How to Get
Earn the Mystical Savior Badge accomplishment badge by completing the Track down the Pantheon and rescue the mystics mission from Laura Brunetti or Miriam Bloechl.
In Ouroboros, as a Flashback mission, you must complete "Find the Pantheon: Rescue the Mystics" which gives the mission "Defeat all Pantheon cultists" in the Champion (Level 25-29) section.
The Mystical Savior badge is awarded to characters of the hero and vigilante alignments. The same badge on villains and rogues is called Mystical Adept.