Astrid's easily detached fairy wings store her magic and enter Carl's inventory after he rips them away.
Rosetta's cookbook note says fairies and sprites store magic in their wings and advises removing them when a direct kill is necessary.
When Astrid freezes Katia at the moment of execution, Carl grabs both wings and pulls. They separate with little resistance, stretching strings of tissue before snapping free.
The removal immediately ends Astrid's spell and drops the fairy to the floor. Carl is surprised by how easily the appendages detach and feels sick while looking at the trailing gore.
He stores the wings in inventory before Katia's crow blinds and kills Astrid. Their later use, value, magical charge, and significance to the Guild remain unknown.
A pair of delicate blood-sparkling fairy wings with severed strands of tissue and stored magic.
The wings are the magic source whose removal breaks Astrid's disabling spell and becomes physical evidence.