The Companion Cube is an item used to summon a Companion Cube pet.
The pet resembles a metal block with a heart in the middle. Just like all pets, the Companion Cube will follow the player indefinitely until they die or the buff is cancelled.
It possesses the ability to activate pressure plates, and emits strange noises similar to various boss deaths when it touches lava.
Occasionally, the Companion Cube will stab the player, but only in dark areas, dealing 1 damage to the player. This can kill the player if they are at 1 HP, giving the unique death message, <player name> was stabbed.
The chance of the Travelling Merchant selling the Companion Cube is 1/10000 making it the rarest item in the game. It is also the most expensive item in the game, costing 5 platinum.
The Companion Cube is an object that can be interacted with from Portal 1 and Portal 2 Developed by Valve.
The strange human-like noise it emits when it touches lava, as well as the tooltip and buff tooltip are all direct references to the same item in the Portal games. A possible explanation for the human noises is from the popular internet theory that humans are stuffed inside of the cube.
It is probably the most or at least one of the most expensive items in Terraria 1.3.1.
The tooltip "Will never threaten to stab you and, in fact, cannot speak." Is a direct quote from GLaDOS after she gives you the cube in chamber 17.