A long time ago, when tthe realm of Nyx haven't even come to existance on Theros, a golden furred Leonin was born on that world, that bore a rare gift. When time passed and the golden cub grew to a strong name that heard to the name of Ceos, his tribe was endangered and he freely volunterred to sacrifice his life for the sake of the people. He held each and everyone of them dear in his heart, so it was an easy call for him. Yet in the moment when it had come for him to die and save his pack, he saw that he had been decieved and his family, friends, his entire pride was been slaughtered before his eyes. He witnissed the malice until darkness overwhelmed him.
When he awoke again he was not on Theros anylonger. He found himself on a waste plain on the world of Alara, long before it fell apart. Yet he didn't know what had happened, he didn't know where he was, nor what he should do. For a long time he wandered the plain, alone and under the star covered firmament of night.
Long he walked in silent solitude, the stars being his only guidiance. He remembered his home, the nightsky of Theros and how he used to gaze upon the firmament with his soul mate and now all but the stars was gone. Again feelings of gieve and loss overwhelmed him. And so Ceos planeswalked for the second time in what should be a long life.
Back on Ceos, he mourned and buried his pride and then he decided that he would further fight the suffering of innocent as long as his spark would burn.
Then eons passed, he leaned to master the powers of a planeswalker of the old days, before the planar chaos. He fought evil on countless planes, yet there was always more darkness in the multiverse. Phyrexia, Bolas, the eldrazi, planar catastrophies, the list was endless.
So with great efford, he created his own world as only the planeswalkers of old could. Vast forests, deep seas, green planes and mountains that reached high into the sky and around his world Ceos created a protective veil of eternal night that would protect this world. A veil that was more then an enchantment, more than just a sky of pretty stars. It bloomed with life and bounded with the world itself. It was a place of peace Ceos had created and he would always be watching over it so at least one place in the vast mulitverse knew immunity.
Yet when Ceos had created Adastra, as he named his sanctum, it was too close to Theros and the enchantment that protected the world of the leonin influenced his native world, what resulted in the creation of the Nyx as it is known today on Theros.

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This is a little lore for Ceos, of course none of that is part of the official canon, but I thought it would fit at least a bit.

Feedback is as appreciated as always, either be it the pic or the story, I would love to hear opinions and feedback as always