I'm trying to find a manga I was reading where they're stuck in a virtual world and they have to fight each other to survive. They can use Magic but if they use too much the power consumes them and they become a monster/go crazy.
It also may not be a virtual world but some sort of castle that has shifting/moving rooms? I also recall the main protagonist (male) having some hidden power.
He travels with a girl who, if I remember correctly, uses too much magic and is about to transform and he stops it from happening.
My memory is pretty bad but hopefully this is enough information to go off of!