Fate
Since Theros: Beyond Death doesn't have a story I decided to make my own.
Klothys was livid.
Her hairs, the strands that connected all things from the heights of Nyx to the depths of the Underworld, were caught ablaze with a fiery light. For the briefest moment all beings, be them god or mortal, felt her wrath, her displeasure at their purposes being overturned, their duties to the world shaken, their well-deserved rest bothered by false hope. All who felt ambition, irreverence or even the mildest temptation towards ignoring their true selves learned in an instance why their aberrant whims would be punished with eternal torment.
And, just as quickly, she was calm.
She extended her spindle, and began to weave extensive tapestries, her horns as the distaff. When she weaved she fully surrendered her thoughts, her creative whims one and the same with all of Theros and sometimes beyond it. For all their pretense of being cosmic forces given form and intent, none of the other gods matched her in this harmony; Kruphix understood the tapestry of fate and Purphoros gave in to his heart in his artistic trance, but both still clung on to their egos, the walls between themselves and all that was. They could not do otherwise, for mortals invison mysteries and awe as invitation to a curious or reverent mind while creation to them requires a rational will. Destiny, however, was at once unpredictable and intended, a concept whose understanding requires a complete disregard of thought and self in favor of complete, unconditional acceptance.
This mindlessness veered close to the hedonism of Xenagos, and the fool was indeed perceptive. But he was still selfish, and Klothys despised him fulgently but so quickly as to not even imply her existence to him.
Klothys had no eyes because she trusted in what was intended and therefore did not need to see at all. Mortals have eyes because they need light to tell them whereas there is danger or pleasure or the darkness of either one's absence, but this has the side effect of making them uniquely vulnerable to blindness. And Klothys was not blind, for she did not fear the twists and turns of life. Indeed, her tapestries glowed in strange and exotic colors, which she could apreciate because she knew they were there and had no ego to require sensory validation.
Some of her tapestries involved the lives of mortals and gods. Some were to die - again in some cases - others were to come to enlightening realisations and stay in the Underworld. Most, however, were agents of fate, either made wholecloth from Nyx itself or from mortals who suspected that destiny was a god. These agents to weave in turn, either strands of her hair she could not reach or simply nets to capture the fugitives. Parts of the tapestry concerned the titans, those putrid amalgations of mutant flesh she had ensnared long ago, now breaching out of their containment in the darkest regions of the Underworld. Like any good writer, she wanted to get to the point but make the events leading to that end glorious.
There was, however, a dissonant knot: Elspeth.
Her fate was intimately connected to the rest of Theros, but she was a planeswalker. Klothys was in harmony with fate to the point she could feel its motions in other planes, but usually they laid far beyond her reach. Her agents were even more limited, incapable of following a planeswalker elsewhere even though many knew of their true nature.
This simply meant making a special agent, one who could follow where others could not.
Weaving that tapestry was a tenuous work, pushing at the very boundaries of her trance to the point she had to pause as to harmonise herself more intensely, an odd mixture between ears wide open and every impulse being enacted. The end result was a being who was so connected to his target that it was his fate to follow her wherever she went, so specialised as to have no other option but to cross beyond Theros into the wide Multiverse. Fate, after all, was the ultimate distributor of sparks, though in this case it meant an additional layer of disregard towards causality and chance alike as to provide a Nyxborn with a spark.
Spawning fully formed from Klothys' taspestry was Calix, the hand of destiny. Clad in green silk and gold ornaments, he was Klothys' masterpiece, a singular display of understanding reality by not understanding it at all. Unlike her he was entirely devoid of passion and unpredictability, but he was completely subservient to his role in the universe. He didn't say a word to her at all: as soon as he came into being he followed Elspeth, who had just reached the shores of Nerono.
Satisfied with her work, Klothys tugged at her hair, spindle soon to drip with blood and starlight.