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.
If you are in the dark with the Companion Cube, the cube will stab you often, dealing damage.
The Companion Cube is an interactable object 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.
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.
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.