There are said to be many of these horrible items hidden throughout the dark corners of the world. They appear as small statues of powerful animals or beasts of various kinds. On the bottom of each is a word engraved in an obscure language. If tested, they will radiate powerful magic. Cursory investigations will indicate that these statues can be placed upon the ground, and when the word spoken aloud by the owner, the statuette will transform into a real-life version of the creature depicted, totally under the owner's control.
In reality, these are magical gates for powerful demons imprisoned in their own dimension, and the words on the bottom of the statues are their names. Activating a statue releases the demon, which will appear immediately, under no one's control. Consult your favorite monster guide for the demon's statistics and powers, but each statue will correspond to a different such being.
Optional: The word on the bottom of a statue may be difficult to translate, requiring a quest all on its own, during which time clues to the item's true nature might be discovered.
Image: Bronze statuette of a pantheress by Metmuseum.org under a Creative Commons Universal Public Domain Dedication 1.0 modified in Inkscape by David