Companion Cube

The Companion Cube is a Pet summoning item sold by the Travelling Merchant for. It summons a pet Companion Cube. When it falls into lava, it emits the sound of the Lunatic Cultist when he has been defeated. It is possible that the cube emits the Whoopie Cushion sound, and damages you, but this is to be further tested.

The Cube can also activate Teal Pressure Pads.

Trivia

 * It is the most expensive item in the entire game.
 * The Companion Cube is a direct reference to the Aperture Science Weighted Companion Cube, from Valve's Portal game series, which is also why the cube makes a sound when touching lava as you are forced to throw your companion cube in an incinerator near the end of the game. The buff tooltip is also a line from GLaDOS, when you get the companion cube, from the same game.

History
Cube de Voyage