Go through the wall, find nothing, head out, but get stuck from the waist down.

Large wall with mysterious hole in the middle of it, the user is primarily female, and able to go through the hole, but see nothing on the other side, and when they're about to get back out, their body at the waist down will change into that of a creature, it can range from anything, and is not limited to earth creatures, not only that, but it doesn't stop at only being quadrepedial. And because of the wall, they won't be able to see the changes, but rather feel them.