If he breaks the seals, Shizuru would straight up try to kill him. And more than likely succeed unless the Dean steps in and tells her not to. There are tens of thousands of yokai locked in there. Breaking the seals would ruin the training area and put everyone at risk. It took incredible magic to seal off the pathways. All the spirits locked in there are fallen soldiers that have become various kinds of monsters in their eternal undeath. Most of them were killed by Shizuru. They'd be understandably pissed.