The King of Equestria, the lover of Queen Majesty and the father of Princess Celestia and Princess Luna.

His relic is the Blade of the Twelve Zodiacs.

STORY:
The story of who would become the father of Princess Celestia and Luna and King of Equestria began when he was just a lonely alicorn. Born from a dying star, a string of energy would eventually take form an alicorn pony and began centuries of wandering alone in the vacuum of space, looking for a partner from different stars. Throughout his long journey, he couldn’t find any… that is until he came across a strong stream of energy from our world. This caught his attention and he descended into the mortal realm to see the source of it… when the white alicorn who would become Queen Majesty defeated the Old Horror Ghroth. Thus, he found a new home that he would be willing to stay for all time… and possibly a new partner.”

“When the two alicorns first meet, he didn’t have a name to introduce himself. So since he came from the star-wonders of space, he donned the name Cosmos. And so the two formed a relationship, growing closer and closer from simply friends to being more than just that. Until, Cosmos received the news that Majesty was missing. It was said that she was last seen heading northeast to rescue a group of missing ponies, so he traveled there also, coming across a dark forest. Thus, he encountered a creature that was called ‘The Black Goat of the Woods’ named Shub-Niggurath. He demanded to know where Majesty and the others, who were actually the Goat’s captives. Cosmos asked why Shub-Niggurath would do this. It replied that it was all alone in the woods and that it had nopony to be with. Cosmos realized that in some sense, he was just like Shub-Niggurath: alone and desperately needing somepony to seek solace with. But he also realized that unlike himself, Shub-Niggurath’s method was wrong, he knew. So he fought the Goat for the freedom of his friend and others.”

“Just when he thought he had won, he was wrong. Soon, Shub-Niggurath unleashed his true form… a colossal horrid ugly abomination that could not be described by mortal eyes. Not only did his form change but so did its magnitude in power, as Cosmos was quickly overwhelmed by Shub-Niggurath’s true strength. But when the odd was very low on his favor, Cosmos declared that he would never stop fighting for the sake of ponies, especially for Majesty, whom he admitted his love for. Therefore, he gathered the energies collected from twelve stars during his journey and created a great sword made of platinum and gold. Armed by the Blade of the Twelve Zodiacs, Cosmos now had the power to finally defeat Shub-Niggurath… and he did. But Cosmos did not kill it. Instead, he imprisoned it in the depths of Tartarus where it would never be alone again but also never wreak anymore havoc.”

“Returning home, Cosmos said to Majesty that his love for her was true and genuine. Majesty admitted she felt the same way for Cosmos. They vowed to one another to love and protect each other as well as their friends and subjects… whom they also saw as family. So, they together became leaders, warriors, partners and lovers… star-crossed lovers.”

NOTE:
I understand that in the show, there are only six Pillars of Equestria. But all this will be in My Little Pony: Revelations of the Apocalypse where it takes place in the Twirageous Saga semi-AU where the point of divergence is around late-Season 3, meaning the events of Season 4-onwards never happened. In this story, there are ten Pillars this time, adding Queen Majesty, King Cosmos, King Grover and Dai Shen Long.

Also, I’m a bit disappointed that we don’t get an appearance or a hint of Princess Celestia and Luna’s parents in Season 7, given the Celestia and Luna’s fully-settled sisterhood in A Royal Problem, the sense of parenthood in The Perfect Pear and the fact that the sisters were young and mentored by Star Swirl. I just hope that we would have a certainty of a hint of the sisters’ parents, the Queen and King, or even better their appearance or flashback, perhaps in Season 8.

This pony is based and inspired from this.
King Cosmos originally belongs to © LugiaAngel