Merlin's Spell Book

 is a feature officially introduced to the game with but available since.

About
Originally, Merlin's Spells unlocked as you leveled up Merlin. At that time Merlin was the only character in a collection that is not playable character, but rather a utility character, and was therefore only intended for that. When he lost his Non-Playable Character status, his spells migrated to this new functionality.