A New Severus Snape

During the Battle of Hogwarts, Voldemort casts a de-aging curse at Harry. Snape gets in the way of the curse, though, which de-ages him seventeen years. Snape is stuck at age twenty after Voldemort's defeat. When Hermione and her friends come back to school in the fall to start their seventh year, they're surprised to find that Snape is de-aged and looking for a cure at Hogwarts.