The Summoner

The Summoner is a magical object that appears in the fifth Goosebumps book, The Curse of the Mummy's Tomb. It is a  mummy's hand that is held as a good-luck charm by the book's protagonist, Gabe Sabry.

History
Gabe bought the hand at a garage sale for $2. The kid that sold the hand to Gabe told him it is called The Summoner and it is used to summon evil spirits. Gabe does not believe that the hand is actually magical, but is intrigued by the possibility that it might be real.

In The Great Pyramid, Gabe learns from Ahmed that the hand is the actual hand of the ancient Priestess, Khala.

Physical appearance
The Summoner is a mummy's hand. Gabe describes the hand as small and says it is the size of a child's hand. It is wrapped up in brown papery gauze.

Powers
The hand makes mummies come to life. Gabe pulls the hand out without thinking when he is in trouble and holds it high up in the air. It makes long-dead mummies come to life and come to his aid. Because Gabe says he was not thinking and does not know why he pulled the hand out, it can be inferred that the hand has the ability to persuade its owner.

Trivia

 * The Summoner is a literary example of deus ex machina, which translates to "god from the machine." In literature, deus ex machina is a device that comes out of nowhere and solves a seemingly impossible problem.