4 Magic Marbles are found within each room of the Wizards' Hollow. 1 Magic Marble in each room contains a crooked Wizard; the other 3 Magic Marbles contain a variety of rewards and allow you to progress to the next room.
There is always a 25% chance of finding a Wizard; this probability is fixed and does not change from room to room.
Magic Marbles in Enchanted Rooms do not contain Wizards. Upon reviving yourself, your current room, as well as the following room, also do not contain Wizards.