The magical mirror-egg spell is a creation Kethry devised based on the concept of the mirror-egg used in Gift training. In this variation, the trap-spell forms an egg-shaped shield around the target, reflecting magical energy back at him. In other trap-spells, the trapped mage just has to patiently search until he finds the keypoint of the spell, and cut his way through it to collapse the spell. That technique won't work here, as any attempt would be reflected back.
With Kethry's spell, there are only two ways to get free--either have someone else break it from the outside, or build up enough pressure inside the shield to overload it. The problem with the second option, is that the mage will get hit with that overload as well, and probably kill himself, unless he's smart enough to put up a second shield around himself to protect himself from the effects of the magical overload.
Kethry originally created this spell to contain King Raschar's mages during the Rethwellan coup attempt. It worked better than she ever hoped. After the coup, she distributed the spell to mage schools far and wide to insure that legitimate mages would be able to recognize it and get help or counter it for someone else if it's used by the wrong people.
The song "Mirror Spell" is about Kethry casting this on Raschar's mages.