Young Indiana Jones and the Mask of the Madman

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Young Indiana Jones and the Mask of the Madman was supposed to be the sixteenth young adult novel in the Young Indy series published by Random House, and the seventh by authors Megan Stine and H. William Stine.

Publisher's summary
Searching for the missing Professor Dante, young Indy and his cousin, Lyle, learn that Dante may be the founder of a bizarre underground cult, and when they find him, they are forced to participate in a grisly ceremony.