Shadow Spies were magical entities that Shadow Weaver created using the Black Garnet. She used them to track Adora and her friends in the episodes "The Sea Gate" and "In the Shadows of Mystacor". The spies could also interact with each other as well as track their victims.
The Shadow spy was very powerful; it knocked out an unsuspecting Glimmer, Bow, Castaspella, and several unnamed sorcerers in Mystacor.