Companion Cube

The Companion Cube is a pet summoning item. It has a chance of being sold by the Traveling 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 eventually kill the owner, displaying the unique death message "" Portable light sources, such as Torches, Mining Helmets or Light Pets, will completely remove chances of being stabbed by the Companion Cube.

When the Companion Cube falls into lava, it emits the Lunatic Cultist's. When submerged in lava for long enough, there is a chance it may make the Moon Lord's. The Companion Cube will also occasionally emit the normally produced by the Whoopie Cushion and its upgrades.

Trivia

 * It is the single most expensive item in the entire game.
 * It is one of three pets that can make noises, the others being the and Glommer.
 * The Companion Cube is a direct reference to the Aperture Science Weighted Companion Cube from Valve's  video game series.
 * 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's tooltip.