This is my first and probably last crossover. It was a bit of a distraction while I write my other fics and no doubt it misses the mark with fans of both shows - sorry! It was so fun to write so forgive my self-indulgence.

The setting for the fight is the demon realm in Nanatsu no Taizai. Zeldris' Umbreon is an alternative evolution of Serena's eevee in the XY anime and the move sets described for both the Umbreon and Gyarados are consistent with the XY version of the game.

In the hush of the night, the demon realm rested, not even a breeze disturbing the quiet peace which had descended over the land. Everyone was in their beds, with only two sleepy guards lazily watching the palace gates, yawning their heads off as they half-heartedly tried to fulfil their duties. But there was no need for vigilance, no risk of invasion. The goddess clan were celebrating the supreme deity's creation and even they would not try any treachery on this night for they were too busy partying in the celestial realm. The whole land and its people felt the benefit of the peace, delighted that for just one night they could all sleep soundly without fear of attack.

Well, almost everyone. Unseen by the guards, two small, lithe figures whipped silently over the castle walls, running swiftly across the grass towards the training grounds, the blades of grass barely bending with the slight pressure of their feet. They were so fast as they sprinted with joy, their bodies picked out in the sliver light of the moon. The demon princes could hardly risk the fight they had planned in the palace itself, their father would throw a fit at the damage they'd cause.

Skirting round the straight rows of the training grounds where swordplay was mastered, the two boys headed towards the circular platform to the right, jumping with ease over the fence which separated the fighting ring from the rows of seats for spectators. Laughing quietly, both reached the centre of the ring, fist-bumping each other before they headed for opposite ends where the trainers would stand. They had been looking forward to this fight for a while and they were going to enjoy it.

Zeldris was confident, his dark eyes flashing with mischief. This was the first time he was going to show his brother what Umbreon could do. He hadn't even let on that his Eevee had evolved yet, let alone the power it had accumulated over the past months. He and Umbreon had trained with determination and he was sure now that she could triumph over anything Meliodas could throw at him. That TM he had found while spying on the fairy king's forest would seal the deal. It was a formidable move, perfectly suited to his dark-powered partner and Umbreon had relished the use of it.

Meliodas smiled. Zeldris was in for a shock. He hadn't even told his youngest brother about the Magicarp he had caught. Why would he? It was such a useless pokémon. But he had not been deterred. When he had caught the flapping gold excuse for a familiar while fishing with Estarossa he had seen its potential straight away. He had made Estarossa battle with him again and again, putting Magicarp into the ring for one move before pulling it out so that his Charizard or Dragonite could take over. And little by little, with the experience of these fights by proxy, the fish had grown stronger until it had finally evolved. So now, after all those months of frustrating effort, he had a shiny Gyarados, a beautiful red and white monster that could annihilate villages. He had tested it out the other day, pleased with the power it had accumulated. The Kingdom of Coventry would never be the same again.

"Okay, the rules are we use one pokémon only, no substitutions. No items, no potions. We fight until one of us can fight no more," Meliodas commanded, his black eyes showing the arrogance of his power. He would not lose, not to anyone and definitely not to his youngest brother, the very weakest of his father's three children.

"Fine with me," Zeldris purred, his face pulled into a smirk. Meliodas was in for a surprise. This would teach his brother not to underestimate him.

Both boys pulled red and white pokéballs from their belts, holding them up so the other could see.

"Gyarados, I choose you!" Meliodas cried and a red mist rose as the pokéball opened, the enormous white-whiskered monolith rising from the depths. Zeldris stared. Since when did Meliodas have this monster to call on?

Zeldris wondered briefly if he should abandon his plans and pick his Vileplume instead. Although it looked like a long, writhing dragon, Gyarados was a water and flying type pokémon and it was vulnerable to grass moves. His Vileplume had those in spades as well as the type advantage and Zeldris knew he should set his pride aside and use it instead. But no. He had promised Umbreon she would have this fight and he would not let her down. What mattered more, beating his brother or keeping a promise to a friend?

And Umbreon was a friend, not just a partner. He had not even caught the Eevee he had found wandering lost in the demon realm, trembling with nerves, unable to eat or sleep with the terror. He had spoken softly to it, coaxing it out of its hiding place, stroking its soft fur gently as it sidled up closer. It had nuzzled into his lap and given way to its exhaustion as Zeldris watched it, his heart full of tenderness as its chest rose and fell in its sleep. Female Eevees were rare, a prize worth keeping for breeding and he had determined to look after it for this purpose.

But somewhere along the way he had lost his heart to the timid little thing. She was so sweet, always pulling the covers off him in the morning to help him wake up and cuddling up to him on freezing cold nights. One day, she had pushed an empty pokéball to his feet and looked at him expectantly, and Zeldris knew then that he could not let her go. She had caught him and he took her everywhere, not fighting with her just enjoying the companionship.

Then one morning they had been walking through the forest up in Britannia and a swarm of Beedrill had attacked without warning. Before Zeldris could reach for his Noivern to see off the pests, Eevee had initiated an attack, shooting the stars of swift into the mass of striped insects. They had scattered, fleeing from his timid Eevee and he had seen the glint of enjoyment in her soft brown eyes. She wanted to fight, so he had trained her in secret. Only his girlfriend Gelda knew of his efforts, letting Eevee fight her Braixen to help her power grow. And one night, after knocking the fire fox out for the first time, Eevee had leapt at Zeldris in her elation, her form shimmering silver as she grew and changed. In her supreme happiness, Eevee had evolved and grown into the perfect, neat Umbreon he loved so much. A pokémon of the night and the moon, her red eyes shinning with confidence and loyalty, the yellow circles on her dark fur so reminiscent of the symbol of his clan. She was one of only two things in the world who truly cared for him and he would not let her down. Not now. Not ever.

"Umbreon, I choose you!" Zeldris shouted and the small, black animal pounced out of the ball and into the ring, landing expertly on her feet as she stared at her opponent. She was no longer frightened. Like Zeldris she feared nothing, sure of her worth. Her master loved her and that was enough.

Meliodas laughed. He was surprised for sure, amazed that the pathetic Eevee which followed Zeldris around like a lost sheep had found the strength to evolve. Well, his brother was nothing if not persistent. But an Umbreon, while more powerful than a timid Eevee, would be a piece of cake to take out. Its attack stats were useless compared to the Gyarados he wielded. Zeldris didn't stand a chance.

"Twister," Meliodas sang with a sneer and immediately the Gyarados opened its mouth, a whirlwind shooting from it straight towards umbreon. Caught off guard, the small animal was pulled into the miniature tornado, squirming fitfully as she tried to find her feet.

"No!" Zeldris screamed. "Keep it together!" and Umbreon looked at him with understanding, jumping out of the wind to land on the ground. Zeldris examined her, she had taken some damage but she would be fine. Her ability, inner focus, had protected her from the more sinister effects of the attack and she had not flinched. She was ready to go.

"Come on Umbreon, use Pursuit!" Zeldris called and the dark-haired beauty rose on her haunches, flicking her power outwards with her claws. Waves of purple flew over the ground towards Gyarados, hitting it squarely in the eyes before it could dodge. The beast writhed, but held its ground as the attack dissipated, barely looking as if it had broken a sweat.

Meliodas sneered, his mouth curving cruelly. "Enough of this. Use Dragon Dance," he snarled menacingly. Instantly, Gyarados started pulling itself up, twisting and spinning in a choreographed frenzy and Zeldris could feel its power crackling across the ring. He knew it was drawing energy to itself, greatly increasing its speed and strength for attack.

"Dark Pulse," he murmured quietly so only Umbreon could hear and swiftly she sent out the dark circles of the attack. Gyarados had increased its speed but not by enough. It was unwieldy and could not dodge despite Meliodas' order, and the attack landed squarely into its rose red body. It shuddered as the Dark Pulse burst over it, throbbing with a menacing black energy. Dark Pulse was a powerful move and Gyarados would definitely have taken some damage this time.

"Hyper Beam!" Meliodas screamed, "Get it now!"

"Dodge it!" Zeldris shouted in warning. If that attack hit Umbreon she would be badly hurt, it was an extremely powerful move. Just like Meliodas to stock up with that. Fortunately, the attack was not hard to evade. As the white hot rod of power shot through the air, Umbreon neatly side-stepped it, moving with unhurried grace to stand a few inches away. Gyarados spun its head trying to catch her with the beam, but she hopped about like a bunny, expertly avoiding it each time.

"Use Dark Pulse again," Zeldris cooed and Umbreon obediently sent out her power, the glowing circles gliding swiftly over the ground. They shimmered as they hit Gyarados and Meliodas hissed with rage as his pokémon faltered, falling back slightly with a fierce snarl.

"Had enough?" Zeldris called, his face alight with enjoyment. It was a rare sight to see his brother so agitated.

"Hardly," Meliodas shot back. He was furious. How could this little thing possibly be winning at this stage of the match? His Gyarados was an unassailable power house, it should be beating this Umbreon into the ground.

"Hydro Pump!" Meliodas screamed and immediately a water cannon of fury shot from Gyarados' mouth onto Zeldris' Umbreon. The pokémon fell to the ground with the force of the blast, her legs twitching pitifully as the water washed over her. Meliodas grinned, his face contorted with a wicked triumph as Zeldris knelt down before his partner, smoothing her fur as he stroked her gently, the spikes of his black hair blending into her dark fur. She was weak, badly hurt, but hanging on for the fight.

"Moonlight," Zeldris murmured softly and the silver orb in the sky was pulled down to the earth to hang over Umbreon as she lay prone on the ground. It shone fiercely, its rays beaming brightly and Umbreon's yellow rings of fur started to glow. She shook herself, standing up as the moon's energy flowed through her veins, partially restoring her drained health.

Meliodas watched with interest. Moonlight, a fairy move. Not one he would have expected his brother to bother with, but it evidently had its uses. He wondered how much health the move had restored. The Umbreon still looked a lot less sprightly than it had before the attack. Even with its restorative power it would surely not take much more to finish it off.

"Hyper Beam!" Meliodas commanded, the sound ringing across the battle field and Zeldris looked up, horrified, as the white power shot once again towards Umbreon.

"Dodge it, now!" he cried, panic evident in his tremulous voice. Umbreon could not take another hit, particularly not from this move. At the last second, Umbreon shot up into the air out of the way of the oncoming attack. She twisted and ran towards Gyarados as she returned to the ground, determination showing in the way her fur bristled and Zeldris had never felt so proud. She was not giving up, and neither would he.

"Last Resort!" Zeldris yelled over the roar of Meliodas' Gyarados as Umbreon continued her charge. She pulled up right in front of the red beast, staring at it coldly before unleashing her power. The ground shook and trembled as the Last Resort hit and the Gyarados shrieked with pain as it fell to the ground. Umbreon trotted back to Zeldris, her red eyes alive with her triumph and he saw her elation in the way she walked.

Meliodas peered at his pokémon and was relieved to see it was not over yet. Gyarados was powerful but its defence was relatively low. It had taken damage from Last Resort but it was still in the game. Meliodas reproached himself inwardly. Umbreons evolved from Eevees, normal type pokemon, and they could learn these surprising atypical moves. He should have anticipated his brother's strategy. But that move had its problems. Zeldris would not be able to use it again until he'd tried all the others and that gave him a chance.

"Dragon Dance," Meliodas whispered into Gyarados' ear. The beast gave him a look of disdain, it was hurt and wanted to sleep, but it reluctantly obeyed. It faltered as it rose up, its head flopping slightly to to one side with the effort as it began its unwilling battle dance. Zeldris stared at it as it squirmed, wondering how much more damage the huge beast could take.

Taking advantage of the lack of attack, Zeldris asked Umbreon to use Moonlight again, restoring her depleted health a little bit more. She was a long way from full strength, but she was more sprightly now, and she proved the point as she dodged Gyarados' Hydro Pump, jumping well to the side of the wave as it crashed towards her.

"Pursuit," Zeldris said with a smile and Umbreon's darkness washed over towards their opponents, hitting Gyarados once again. It stumbled back, floundering as it tried to stay upright before wilting slightly under Umbreon's glare. Meliodas sent out Hyper Beam, and this time Umbreon countered it, her Dark Pulse meeting the beam in the air. The two powers clashed forcefully together, white and purple sparks falling all around like fireworks as the moves broke apart on contact. Both pokémon fell back slightly with the effort to stare at each other from opposite sides of the ring once again. The match was drawing to its close. Neither protagonist could go on much longer.

"Hydro Pump!" Meliodas screamed as he went in for the kill, and once again Gyarados obeyed, but it was reluctant, the move shooting out much more slowly than it should. Zeldris stared. This was interesting. Had Meliodas not ensured himself of this beast's loyalty before he tried to use it? Umbreon gave Zeldris a look, taking the signals of his hands as her instructions. They had trained so hard, so fiercely, that they were now almost as one and she could follow Zeldris' unspoken commands.

Umbreon ran around the outside of the ring, followed by waves of water as Gyarados tried unsuccessfully to catch her until she was standing directly behind the beast. With a howl of triumph, she let forth Last Resort straight on to her opponent as it whirled round to catch her, water spewing in all directions as it twisted and squirmed.

The attack hit its mark and Gyarados sank to the ground, groaning loud as its enormous body hit the earth, the ring shaking with the force of the impact. Its pupils were gone, replaced with mere swirls of black, showing that it was well and truly out for the count.

"Gyarados, return," Meliodas said softly as the red mist fluttered over the pokémon's body, pulling it back into its pokéball. "You fought bravely," Meliodas murmured soothingly as the light of the ball went out, signalling Gyarados' safe encasement. "We'll train harder. You'll learn to trust me and we'll get them next time."

Umbreon leapt into Zeldris' arms, and he held her close as they shared the joy of their triumph. Reaching into his bag, he pulled out a max potion which he spayed over Umbreon, the blue mist healing her wounds and restoring her health. He always took care of her before attending to the post-match pleasantries. Meliodas approached, his hand stretched out and Zeldris took it in a firm grip, trying hard to keep the smirk off his face. That would teach his elder brother to be so self-assured.

The three of them walked slowly back to the palace as the sun rose tentatively in the east, its golden rays lighting the lush vegetation of the demon realm. Umbreon walked close to her trainer, her body brushing against his legs as he reached down to ruffle the fur on her head. Zeldris smiled. Sometimes supreme power wasn't everything, even if his family prized it above all else. Trust, friendship and love separated the winners from losers he thought as he gave Umbreon a gentle scratch behind her black ears.