The Black Cube of Darkness is a seemingly ominous object who was considered to be concentrated evil and only speaks in Latin with a deep demonic voice. He was formerly one of the most powerful and evil villains in the galaxy and had dozens of planets under his control, until gave up villainy after failing to acquire the "Ring of Invincibility" in a humiliating event. He has since reformed with the aid of Wander and found a new lease on life.
The Black Cube was a being of pure evil who terrified and stole the souls of those around him and spreading his influence throughout the galaxy. He wouldn't hesitate to become even more powerful if given the chance. Despite this and the fact that he is pure condensed evil in a cube, the Black Cube of Darkness is actually quite self-conscious about being a cube and becomes incredibly saddened when realizing that he has no limbs or a proper living body like other beings in the galaxy. Eventually this self-conscious attitude led to severe depression, which showed that that the Black Cube was actually a sensitive being who wasn't purely evil.
The Black Cube destroys his enemies on a deeper level than physical because can steal people's souls simply by speaking to them. The only way to avoid having one's soul sucked is to completely avoid eye contact with the Black Cube. Victims of the Black Cube's power are rendered unconscious after looking at him and their eyes will become pitch black as their souls are stolen, however the Black Cube has the ability to return them as well, but his victims seem to be slightly drained and confused afterwards. The Black Cube can only steal a soul if he stares at a victim for a prolonged period, making his powers truly dark and terrifying and alsho has telekinetic powers which he once used to crush a cash register, but he might only be able to use them when truly angry, as he doesn't use this power to touch or move things, instead moving or pushing them with his own body most of the time as though he had hands, which he doesn't. His body is also seemingly indestructible as shown in "The Battle Royale", where General McGuffin's missile was rendered bent and useless after coming in contact with the Black Cube. In "The Battle Royale", its power to steal souls was on par with Little Bits' power to control them and both were seemingly immune to each other's influence.
In the Battle Royale, the Black Cube was battling Little Bits for the ring of invincibility, and later on, it reached the ring, but failed to obtain it due to it lacking any hands. Its fellowed villains look on with shame, disappointment and laughter which causes the Black Cube to sadly floated away as he realized his limitations.
More later, in "The Black Cube", after suffering his most shameful loss when he failed to obtain the Ring of Invincibility, the Black Cube could no longer go on as a villain and quit, eventually moving into a small apartment on some city planet where he got a job as a cashier at a fast food restaurant. The Black Cube got himself a girlfriend either during or prior to these events. Eventually though his girlfriend left him, his rent was due and his car was towed, resulting in him almost losing his temper. Luckily Wander appeared and cheered the villain up and told him to be positive. This worked for a time, but after running into a trio of bullies and several freaked out citizens who still thought he was a villain, the Black Cube grew depressed once more, with Wander trying to cheer him up once more, only to fail due to the intervention of the bullies, who then got him fired. The Black Cube then floated around town in the rain to do some soul searching in hopes of finding what to do with his life. He was then harassed by the bullies once more, but after one falled off a bridge, the Black Cube attempted to save him, only for the bully to accuse the Black Cube of pushing him. An angry mob of citizens then formed to seemingly kill the Black Cube which almost causes him to unleash his rage on them. Wander then intervened and tried to calm him down only to almost have his own soul sucked out. Sylvia then reprimanded the Black Cube as well as the citizens for letting their anger get out of control and that they should learn to forgive and forget. The citizens and the Black Cube realized their mistake and apologized. Wander was then revealed to have survived and everyone began singing that they're all Little Black Cubes of Darkness, which caused the Black Cube to shine brightly as the rain stops. Wander then dried off some water from the Black Cube's face, leaving behind an image of a happy face with a tear of joy.
The Black Cube of Darkness is seen in "The End of the Galaxy" to aid Wander and a group of rebels to fight Lord Dominator.