Companion Cube

The Companion Cube is a Pet summoning item sold by the Travelling Merchant for. It summons a pet Companion Cube that will jump around the player frantically.

As an easter egg, when it falls into lava, it emits the Lunatic Cultist's death sound. When submerged in lava for long enough, there's a chance it may make the Moon Lord's death sound. Surprisingly enough, the Companion Cube will occasionally make the sound of a Whoopie Cushion. A similar behavior is that the cube will "stab" its owner for 1 damage. The stab can kill the owner if at 1 HP, with the unique death message "[Player name] was stabbed." Although, the Companion Cube will only attempt to stab the player if both the owner and the Companion Cube are in a non-lit area (and light pets have been extinguished).

Unlike other pets, the Cube can activate Teal Pressure Pads, which gives it potential for adventure maps.

Trivia

 * When sitting in a thin amount of lava (like when the cube is mostly exposed to air, except for a small portion) and the player is standing still, the cube (when sitting in the lava) will bounce, disappear, then reappear (instantly) where the player is standing. If in the right place, it will continue to loop.
 * It is the most expensive item in the entire game.
 * The Lunatic Cultist sound will only play a maximum of once per minute, regardless of how many times the cube touches lava.
 * The Companion Cube is a direct reference to the Aperture Science Weighted Companion Cube, from Valve's Portal game series, as is the fact that the Companion Cube can activate Teal Pressure Pads. When GLaDOS disintegrates a companion cube in Portal, it emits a death sound. The Buff tooltip is also a line of dialogue from the first game.
 * The sounds it make may be a reference to the internet theory that the cube contains a human body.

History
Cube de Voyage