The Haunted Mask by R. L. Stine

A shy girl fed up with being frightened buys a grotesque Halloween mask that, once worn, inexplicably fuses to her face and amplifies her worst impulses, transforming her into a mean, terrifying version of herself; as friends and family recoil she desperately tries to remove the mask, uncovers the dark origin and power behind it, and must confront her own fears and choices to break free and reclaim who she really is.

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