To be used for the final encounter, find the final treasure, or anything that needs to be hidden away better then normal. Typical “who the heck would ever spend the time making it” but it is fun and a mental challenge for a change.
Then suddenly all your body paralizes and you hear a voice echoing in the distance "seems like another rat has been lured to my maze," this voice was terribly cold but playfull, "come on you are not already planning on leaving, the fun has only begun."
Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or "bounce back". The grater the magic, the more it should cost the character physically, emotionally, and spiritually.