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.

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