In Summer Knight, chapter 27:

I squinted at my surroundings next and found a ring of toadstools grown up around me. They weren't huge, tentacular, horribly fanged toadstools or anything, but it put a little chill in me all the same. I lifted my hand and reached out for them tentatively, extending my wizard's senses along with the gesture. I hit a wall. I couldn't think of another way to describe it. Where the ring began, my ability to reach, move, and perceive with my supernatural senses simply ended.

 

Trapped. Double huzzah.

This was naturally, a prison for Harry made by one of the fae after having been duped into fetching the Unraveling for the big bad fae of the book.

In Summer Knight, chapter 27:

I squinted at my surroundings next and found a ring of toadstools grown up around me. They weren't huge, tentacular, horribly fanged toadstools or anything, but it put a little chill in me all the same. I lifted my hand and reached out for them tentatively, extending my wizard's senses along with the gesture. I hit a wall. I couldn't think of another way to describe it. Where the ring began, my ability to reach, move, and perceive with my supernatural senses simply ended.

Trapped. Double huzzah.

This was naturally, a prison for Harry made by one of the fae after having been duped into fetching the Unraveling for the big bad fae of the book.