While traversing the Petrified Forest, Manny Calavera encounters this cryptic message:
They'll tear you apart, bone by bone...
...and build with you a human throne.
Their buck-toothed king will sit upon
What once was you but now is gone.
This key unlocks the gates of Hell.
Steady traveler, use it well.
Though he doesn't realize it at the time, this is meant to warn him of the nearby Demon Beavers. Finding a way to bypass them is one of the key puzzles in the game's first chapter.