Once long ago the queen of the Light Kingdom was blessed with twins.
It was decided that, instead of bequeathing the full visible spectrum to one, only half the colors would be gifted to each.
The first of the twins born, a boy named Khruson, was given power over the colors white, magenta, red, and yellow. His cradle was made of mahogany and decorated in pearls, amethysts, gold, and rubies.
The second of the twins born, a girl named Ariane, was given power over the colors gray, green, cyan, and blue.  Her cradle was made of bamboo and decorated in silver, emeralds, sapphires, and jade.

Nearby in the Empire of Shadow the dark queen was also blessed with child, a son named Oura.
Oura was given domain over the black and the invisible things of the world.
His cradle was made from cypress boughs and was decorated with iron, diamonds, and onyx.

Over the next two decades Khruson grew into a handsome prince. Long red hair flowed out from beneath his gold crown, which was inlaid with a symbol of the sun.
Khruson was strong and courageous and had great respect for tradition, and for authority, and for sacred purity.

Ariane had likewise grown into a beautiful princess. The crown that rested upon Ariane's silver hair was inlaid with a symbol of the moon.
Ariane was swift and intelligent and had great respect for fairness, and for justice, and for truth.

Oura was also handsome, though his pale skin seemed waxy and his black eyes were sharp and piercing.
The crown that rested upon his curly black hair was cast with a few small diamonds but mostly bare, to symbolize the dark void of interstellar space
Or maybe he just had a symbol of the earth.
Oura was sly and calculating. He valued self-reliance, and patience, and most of all, pain as a tool to cut out weakness.

It was in this time that a wandering sage presented the Light King with a prophecy whose telling his dreams had long anticipated: the king would soon die.
It was now upon the king to decide whether he would divide his great kingdom equally among his children or let one reign supreme and alone.
The king did not wish to separate his people or break his nation's strength, but neither did he wish to unfairly favor one of his beloved children.
The king decided to give each child a quest into darkness, and his land would be given to whoever returned, or split among them if they both did.
If neither could stand against the darkness, his kingdom would fail.

Khruson was given an enchanted ax of gold that could cut through any non-magical substance, though it became heavier and heavier the faster he swung.
Khruson was told to take his ax into the forest and cut down a tree.
He would see where it fell, and cut another in that direction, over and over till he found a tree which he could not cut. After that he would know what to do.

Ariane was given an enchanted hunting bow of silver. It could shoot arrows as accurately as she could see, but drawing it became more and more difficult the nearer she aimed.
Ariane was told to take her bow into the fields and shoot an arrow at a cloud.
She would follow the dying cloud as it drifted to the earth, then shoot another arrow in that direction, over and over till she found one her arrows could not kill.
Then she would know what to do.

In the land of shadows, the dark queen's agents had captured the wandering sage.
The queen tortured the sage for the information she had long suspected, and then began preparing her son Oura for his rise to power.
Oura was given an enchanted spear with a long sturdy hilt of iron that could grow as long as he wished to stab, through it also became heavier and harder to direct.
Oura was told to walk in an ever widening circle from their camp, always pounding at the ground with butt-end of his spear. When the ground gave way, he would know what to do.

Khruson had been cutting for many hours when one of the fallen trees fell into a stream. He followed after it until it was stopped by the edges of a rocky ravine which the stream had cut.
Khruson climbed over the tree and looked around. The sedimentary rock walls were speckled with green of tarnished native copper, which likewise had turned the deep clear river water green.
It was beautiful and Khruson wished his sister Ariane, to whom the colors rightly belonged, could see it.
Further down the river Khruson found the tree which he could not and would not even dare attempt to cut, for it was adorned with brilliantly sparkling red and yellow apples.
He took one down, bit into it, and suddenly knew what to do.

Eventually Ariane's arrows led her past the field to the snowy top of a mountain. The next cloud she shot, instead of falling, seemed to shake with laughter.
Then the vapor spirit sped toward the mountain and disappeared into a a cave entrance. Ariane stashed the silver bow at the holster on her blue robes and following on swift feet.
Inside the cave, rock formations of red and white gypsum and orange and brown sandstone were illuminated from holes in the ceiling high above and from the reflections of pools far below.
It was beautiful, and Ariane wished her brother, to whom the colors rightly belonged, could see it.
Further into the cave Ariane found a tree, bathed in soft white light that was seemingly radiated by the air itself, adorned with brilliantly sparkling green pears.
She took down a pear, bit into it, and suddenly knew what to do.

Oura had been walking in a slowly unwinding spiral for almost a half a day, pounding the shaft of his spear at the soft ground of the swamp around his home, when he met with a satisfying thud.
Oura struck again to test, and heard a clang. He struck once more, this time with all his strength, and the ground below him gave way.
Oura fell down through a long bedrock tunnel and landed painfully on his rump.
The darkness around him was near total, save for a few glittering charcoal colored Hawthorn fruits on a great a tree that had no business growing there.
Oura grasped for a handful of small berry-like pomes, threw then into his mouth, and suddenly knew what to do.

Khruson and Ariane become a) a mouse and rabbit, b) an eagle/phoenix and a cougar, c) a lion and a bear? I like a) the best, but it's hard to write a fight scene like that. Let's try b) and see how it looks.

Khruson felt a tingling that started at his heart and spread through his chest and abdoment, this his whole body till it reached his fingers and toes and the dangly parts of his ears. He began to glow a soft ruddy orange. Khruson could feel the Dark Empire far away, pushing into the Light with claws and tentacles and arms of smoke. Khruson closed his eyes and focused on the tingling. As he did, the light grew more brilliant. Then he turned into a fire bird, which is totally a thing and also possible. I asked a scientist, you don't even know.

Ariane used the magical powers of the pear to turn into an ice cougar, which is also a thing. No tingling sensation, she just turned into a cougar.

Oura used the magical powers of the Hawthorn fruit to transform into Sud Harazh, the water leaper.
The Sud Harazh was a great swamp demon, with fleshy wings like a tattered black cloak and a heavy black stinger at the end of its powerful tail that glinted like metal.
A Sud Harazh emerged once a century from the hell fires of the earth's mantle, up through a hole in the earth's crust. Just so now, Oura rose from his hole and flew toward his prey. But then he stopped short at sat at the edge of the Dark Empire, because a battle would be more dramatic there and he preferred waiting over stalking.

Ariane and Khruson could feel the darkness in the world, like a hole in their abdomens. They met together at the swamp where Oura had settled.
The sud harazh flapped his powerful wings downward, gliding back and forth laterally across the water in a slow tauting dance. He flicked his heavy black stingered tail insolently and licked at his sharp black teeth with with a forked tongue. Oura waited silently for their attack. Or maybe he just sat there.
Can he become invisible? Can he shrink in size? Can he blind them with shadows? Metal teeth, claws, and and a tail. Not a poison stinger, just a blade on his tale.  What do I call that?

Khruson melted a rock into magma, and Ariane formed it into an arrow tip and an axe blade. Khruson evaporated the water, and Ariane fashioned it into arrows. Khruson snatched down a tree with his sharp talons and nipped it carefully with his beak. Ariane used her cougar dexterity to make it into a bow and an axe hilt.  Khruson shed his feathers for the arrow fletchings. Now they had weapons. And don't they feel silly, because they already had enchanted weapons with built in weaknesses to make the story interesting, and now they have crap weapons which they can't even use because they don't have thumbs. Okay skip the weapons. Or just give each animal one power.

Clouds overhead are shot down again and trees are felled?

Possible Endings?
1) All three rule together in a triumvirate, with Oura taking the ultraviolet and infrared portions of the spectrum. The light twins deal with secular matters and Oura deals with religion.

2) Ariane and Khruson defeat Oura but refuse to kill him. He goes back to the Shadow Empire and begins preparing for his daughter's rise to power. Planning such great pain required great patience, and Oura was very patient.