Ousama no Propose

A powerful and beautiful witch named Saika Kuozaki is the only one capable of stopping world-ending entities that appear every 300 hours. After she is critically injured, she transfers her powers and body to Mushiki Kuga, an ordinary high school student who finds her. Mushiki now possesses her abilities but lacks control over them. To help save the world, he must attend a mage academy while hiding his true identity.