The Curse of the Mummy's Tomb by R. L. Stine
Gabe just got lost - in a pyramid. One minute, his crazy cousin Sari was right ahead of him in the pyramid tunnel. The next minute she'd disappeared. But Gabe isn't alone. Someone else is in the pyramid , too...Someone. Or something. Gabe doesn't believe in the curse of the mummy's tomb. but that doesn't mean that the curse isn't real...Does it?