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 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 also tends to make the sound of a Whoopie Cushion when it's owner is off-screen for others around them in multiplayer. This sound is client-side, and can only be heard by other players on the server. A similar behavior is that the cube will "stab" it's owner, making them appear to lose health to other players whilst not actually being injured on their side. 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.

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

Trivia

 * 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. The Buff tooltip is also a line of dialogue from the first game.

History
Cube de Voyage