Disclaimer: I don't own the Avengers or any other story or character created by Marvel Comics and used in the Marvel Cinematic Universe (MCU). Stan Lee created these characters. I only use them for entertaining purposes and not to be paid for this story. Marvel Worldwide Inc., also known as Marvel Comics is an American publishing house created in 1939, it used the name Timely Publications at first. Some of its characters that belong to the superhero genre are: Spider-Man,Captain America, Hulk, Fantastic Four, Iron Man, Wolverine, Thor, Deadpool, the X-Men, the Defensors and the Inhumans.

This being said, let's begin with the actual story

Nothing and everything at once. Emptiness and fullness coexisting together. Opposites that repelled and attracted each other to their respective counterpart but without putting the other down because they know that they're nothing if one stops existing. Blue and Red that were as identical and different as Ice and Fire. A perfect cosmic dance that provided their souls with the balance they desperately craved for. A dance that defined the whole melody and rhythm from which the universe itself was created and could end it too if, by mistake or a whim of destiny, one step was taken the wrong way and therefore, it causes one of the two primordial forces to cease its existence or changes the rhythm, turning it into an entirely different harmony that has nothing to do with the initial rhythm and order.

The spark that kept her magic doing its miracles and tamed her wild emotions, beasts and demons that were fighting to be released as soon as she let her guard down is extinguished now and it would never ignite again, leaving Red completely so cold, empty and drained of all life that the Arctic, a black hole and cloudiest and greyest of days were warm, full and colourful compared to how dull looked and felt Red, who was supposed to be the magic, the spark, the little miracle while Blue was supposed to be the speed, the logic, the brains and the one who kept her demons locked away in the deepest corner of her soul, so nobody would ever know what lurked behind the "innocence" Red had and how broken she really was.

He died and with him, her sanity and anchor to reality, the glue that kept her shattered soul in one big, deformed and broken piece and his absence only left an eternal abyss of blackness that no one would ever fill because there was nothing that could fill a bottomless void that only expanded itself, separating the shattered pieces now that the glue wasn't there anymore, a whole universe was about to be born inside of Red and it was swallowing and engulfing any ray or spark of light that dared to defy its gravitational force. The light to her shadow, a shadow that once had multiple hues since it used to be hit with different rays of light, each one with different levels of luminosity that varied from being blinding white to being a grey so dark that it could also be a lighter hue of black. Now, all that's left is an endless, piercing, dense and pure black shadow that would never again be so multilayered, that shadow made a night sky without stars look like the brightest dawn.

He, who once was blue, is now a crimson red. The intensity and brightness of that red was the sickest of all the possible jokes that life could have ever made to her, its vivid hue is now a constant reminder of his life being drained and stolen from him. She never imagined that a corpse could be so colourful, it was almost as if he was still alive... only that it wasn't the case and Red would have a completely different meaning to her from now on. Red would be now her reason to live and die at once for the rest of all her meaningless, cold and monochromatic life. Black would be the color of her life since Blue was gone because of Red and she didn't feel like being Red anymore, the void that would eventually claim her whole being was the blackest thing that has ever been witnessed by the entire universe.

Black was bottomless and infinite, it was everything and nothing at once, it was mysterious since you never knew what lied in or behind it, it was uncertainty incarnated and nobody would ever know what to make of it and its nature. Black had all the answers she could ever want and need and at the same time, it didn't have any answer she could possibly want or need to have. Black was her last chance to have equilibrium since Blue died and with him, Red ceased to exist in order to become Black and do what neither Red or Blue could do: Stay together for all eternity.