Legends of Sorcery by Julie Trettel

In the thriving kingdom of Gavalon, filled with lush green lands, waterfalls, and majestic unicorns, magic is still alive and well guarded by the High Keeper.
The clumsy, unassuming orphan Arion has been chosen to be the High Keeper’s apprentice, but it comes with great personal sacrifice. He must take a vow of celibacy dedicated to the study and protection of magic.
When Arion finds an ancient stone relic in the Hall of Magic, dark magic is released, and strange things start to happen across the Kingdom as the unicorns begin to fall to an unknown illness.
Arion is sent to investigate to help find a cure if they are to save the magical beasts that he’s vowed to protect and care for. If the disease continues and the unicorns succumb to it, all magic in the kingdom could be lost.
But a chance encounter with the beautiful Princess Valaria could alter his life’s path forever.
The clock is ticking. Can Arion and Valaria work together in time to save the Kingdom or will magic in Gavalon be lost forever?