The Ultimate Book of Spells (2001)

S1E18 Veil of Illusion

In this episode, the central conflict revolves around the vanishing "Veil of Illusion" at the school, and surprisingly, Morties are able to see it! They soon discover that a powerful wizard is pilfering it to conceal his fortress from Zarlak. The trio realizes they must retrieve the veil to restore order.