"Another mage." One he's not seen since. He glances over at her briefly. "Thorne seems to have it active in the dungeons at all times. Both times I was locked up, the spell was present. But some of its mages are able to move freely without being affected."

He does not explain what he means by both times. For the conversation, it isn't relevant. The point is that it doesn't appear to be a strenuous enhancement to cast or maintain for the level of magic which Thorne operates at.

As for the Free Cities—he imagines they must have or are attempting something of the same. Possibly infused in a device rather than a traditional spell. If one plans to go to war with a kingdom full of mages, it stands to reason they would consider how to nullify magical abilities.