Companion Cube

The Companion Cube is a Pet summoning item. It has a 1/150 chance of being sold by the Travelling Merchant for. It summons a pet Companion Cube that will jump around the player frantically.

When the player and the Companion Cube are in complete darkness, the Cube can occasionally "stab" its owner, inflicting 3 damage. The stab can kill the owner if the player is at low HP, with the unique death message "[Player name] was stabbed."

When the Companion Cube 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. The Companion Cube will also occasionally emit the flatulence sound normally produced by the Whoopie Cushion and the items crafted from it (e.g. Fart in a Jar).

Like other pets, the Cube can activate Teal Pressure Pads.

It is one of two pets that can make noises, the other being the.

Trivia

 * It is the single 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, as is the fact that the Companion Cube can activate Teal Pressure Pads. The Companion Cube reacting to lava is a reference to how the Companion Cube is disposed of in Portal, by being dropped into an incinerator.
 * The Companion Cube stabbing the player is a reference to a line spoken by GLaDOS in Portal, in which she states "The Enrichment Center reminds you that the Weighted Companion Cube will never threaten to stab you and, in fact, cannot speak." This line is also directly quoted in the Buff tooltip.

History
Cube de Voyage 동행 큐브