The Contract Between a Specter and a Servant, Vol. 5 (Light Novel) - Michiru Fushino

All he wants for Christmas is to be curse-free! Masamichi is truly enjoying his life, working alongside Shino at the antique shop. Although the gorgeous specter comes across as blunt and cold, he's actually quite kind, and Masamichi has developed feelings for him. Their first Christmas together approaches, so while Shino is on a business trip, Masamichi goes shopping for festive supplies. But he is suddenly ambushed by Kagiroi, the specter who nearly killed him and Shino in Kyoto! Long ago, this evil spirit murdered Shino's master. Now, the dangerous creature has his eye on Masamichi. In an act of malicious mischief, Kagiroi casts a spell on the young man, transforming him into a five-year-old kid!