In the Marvel Cinematic Universe, the Cosmic Cube (also known as the Tesseract) was sought after by the Red Skull to power his conquest over the entire world with his Nazi organization HYDRA. In the present day MCU, it was revealed that the Tesseract was in actuality an Infinity Stone. Thanos would go on to collect the Stone in the first scene of Avengers: Infinity War, the second on his way to collecting and building his Infinity Gauntlet. However, in the Marvel Comics Universe, the Cosmic Cube is actually its own cosmic item, and has reality-bending powers in its own right.
The Marvel Origins of The Cosmic Cube
The Cosmic Cube made its first appearance in Marvel Comics back in the pages of Stan Lee and Jack Kirby's Tales of Suspense #79. In its first debut, the Cosmic Cube was a creation of Advanced Idea Mechanics aka A.I.M. However, in the 80th issue, the Cube was stolen by the Red Skull, who sought to use the Cube to become emperor of the world. While Captain America was able to keep the Cube away from the Skull, a quest for the Cosmic Cube began which would continue over the course of Marvel Comics' history. The power of the Cube has been eagerly sought after by several organizations like HYDRA and A.I.M., and even SHIELD has been known to seek its power (just like in the MCU) though typically to disastrous results.
In the comics, it was revealed over time that there wasn't just one Cosmic Cube, but several. Often defined as matrixes, the Cubes housed untold amounts of energy and power believed to be originally sourced from the mysterious cosmic beings known as The Beyonders. More often than not, their power is just too much to handle, sometimes driving even the most intelligent characters such as AIM's MODOK to insanity when he tried to create a Cube of his own.
Red Skull is especially obsessed with harnessing the power of the Cubes, just as he is in Captain America: The First Avenger, and would go after them several times. As a result, Captain America is probably the hero in Marvel Comics who has the most experience with the power of the Cosmic Cubes.
What is The Cosmic Cube's Power, How Does it Work?
The Cosmic Cubes are defined as having near unlimited power, with many having the ability to warp reality to the holder's whim as they see fit. Even fragments of the Cube have enough power to be deemed a serious threat to the natural order of the world. There was even a time when Red Skull was so insistent on having the Cube's power that he took fragments and pieces of multiple Cubes to create one whole Cube, though it didn't go so well. Thanks to the makeshift Cube, his consciousness ended up being set adrift and became trapped inside the mind of the Russian general in charge of the Winter Soldier.
The first Cosmic Cube made any of the holder's wishes a reality, while also being instrumental in the creation of A.I.M.'s Super-Adaptoid. While the appearance of a second Cube initially seemed powerless and defective for quite a few years, it too gained the ability to reshape reality before exploding due to its volatile nature. Over time, it was also revealed that the Cosmic Cubes don't have to be Cubes, and they could be any kind of geometric shape in their construction, thereby making them Cosmic Containment Units as a more general term. Furthermore, it's been revealed that these Cosmic matrixes can even gain self-awareness and become sentient beings.
Most recently, the most notable uses of a Cosmic Cube was by Maria Hill and a little girl named Kobik. Kobik was actually the manifestation of several different fragments of Cubes that SHIELD was trying to harness as a means to reshape reality when they deemed it necessary. Despite the fragments becoming Kobik, she still has the power of a Cosmic Cube. Maria used the girl to create Pleasant Hill, a small out of the way town of their own design where SHIELD would dump supervillains after Kobik brainwashed them to become harmless civilians to fill the town. Naturally, this idea eventually fell apart when the Avengers found out, as they saw the lack of ethics in Maria's strategy.
However, Red Skull got his hands of Kobik in the fallout, treating her with perceived kindness. As a result, he was able to convince Kobik to reshape reality so that Captain America was replaced with a doppelgänger Cap who grew up as a secret Hydra sleeper agent. This led to the Secret Empire event, where this dark Cap took over HYDRA from Red Skull and used it to take over the entire world. Eventually, the real Captain America convinced Kobik to help him and let him back to the real world, after he showed her all the pain that Hydra Cap had caused.
The Cosmic Cube Was Re-imagined in The MCU
Obviously, for those who have seen the MCU films, the Cosmic Cube (or Tesseract) has a much more streamlined origin and powers. In the MCU, the Tesseract made its first appearance in the first Captain America film, in the hands of the Red Skull during WWII. Skull used the Tesseract's energies to power the weapons and vehicles of his organization HYDRA, and was planning to bomb major cities with its power as well before Cap intervened. When the Red Skull held the Cube in his hand at the end of that film, it transported him to the cosmos while the Cube was recovered by the SSR and later SHIELD.
Decades later, Loki came to Earth in the first Avengers film, seeking to take and use the Cube to create a wormhole in space for his master's armies to come through and invade the planet though they were defeated by the Avengers. Loki's master would later be revealed to be Thanos, the Mad Titan, who knew the Tesseract's true nature and state as one of the 6 Infinity Stones, more specifically the Space Stone. Thanos would eventually get his hands on the Stone in Avengers: Infinity War, using it to eliminate half of all life in the universe. However, when the Cube was still in SHIELD's hands, it granted Carol Danvers with powers of her own, and she became Captain Marvel.
Obviously, the Cosmic Cubes are just too powerful for anyone to wield, though it seems like there will always be someone in the Marvel Universe that will try and claim its power for their own. It makes sense why a Cosmic Cube only shows up every once in a while in the comics. Anything can happen when they come into play, and they usually shake up the status quo in some pretty big ways.