Companion Cube

Type	Pet summon
Use time	20 (Very fast)
Tooltip	Susceptible to lava!
Rarity
Buy	5 PC
Sell	1 PC
Research	1 required
Buff	Companion Cube(Desktop, Console and Mobile versions)
Buff tooltip	Will never threaten to stab you and, in fact, cannot speak
Use	https://terraria.wiki.gg/wiki/File:Item_8.wav

Desktop/Console/Mobile-Only Content: This information applies only to the Desktop, Console, and Mobile versions of Terraria.

The Companion Cube is a pet summoning item. It has a chance of being sold by the Traveling Merchant for 5 PC. It summons a Companion Cube pet 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 "<Player name> was stabbed." Portable light sources, such as Torches 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 death sound. When submerged in lava for long enough, there is a chance it may make the Moon Lord's death sound. The Companion Cube will also occasionally emit the flatulence sound normally produced by the Whoopie Cushion and its upgrades.

Notes

The Lunatic Cultist death sound will only play a maximum of once per minute, regardless of how many times the cube touches lava.
The Companion Cube, if in complete darkness, does not need to touch the player to stab them.
The Companion Cube will move and bounce repeatedly through solid blocks to stay close to the player if the player is more than 6 tiles away.
When sitting in a thin amount of lava and the player is standing still, the cube (when sitting in the lava) will bounce, disappear, then reappear instantly where the player is standing.
The Companion Cube's attack can be dodged using Master Ninja Gear or other similar dodging effects.

Trivia

The Companion Cube stabbing its owner. Note this only occurs in complete darkness.

It is the single most expensive item in the entire game.
- It also has the highest sell value of any item in the entire game.
It is one of three pets that can make noises, the others being the Pet Worm(3DS version) and Glommer.
The Companion Cube is a direct reference to the Aperture Science Weighted Companion Cube from Valve's Portal 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.