"Alright, Gengar. Are you ready?" The trainer asked with an inviting smile as he took a few steps back into the clearing, looking at his pokemon. His Gengar cackled joyfully while gripping tightly on the Gengarite they had found earlier that week, nodding vehemently. They had been waiting for this moment for so long, it was finally time to unleash the ghost's full potential.
"Here we go, then. Gengar, mega evolve!" The trainer shouted, raising his arm and tapping the key stone he carried within a bracelet. The crystal immediately burst into orange light at the same time as the Gengarite burst into a light blue light, considerably brightening the dark clearing. Both stones launched bolts of colorful energies towards one another, which connected and changed into a dark purple energy that recoiled into Gengar and engulfed him.
The ghost started changing shape right before his trainer's eyes, his back spikes growing far larger and sharper and showing up around his arms and legs, which also grew. Gengar wailed out in agony from the overflow of energy surging through his body, causing the trainer to wince. The first time was always the worst.
The dark energy exploded after about a minute, fully revealing Gengar's new form. Aside from the new, enlarged spikes, his face had changed into a darker shade of purple, which gradually changed into a red hue as it went down the ghost's body. His legs were still stubby, but his arms had grown considerably, in an almost comical way. As soon as the trainer observed that, however, Gengar's limbs sunk into the ground, leaving the body halfway visible. The ghost was panting heavily, eyes shut as her recovered from the strain of mega evolving.
"We did it!" The trainer exclaimed excitedly, running to his partner and kneeling down by the mega evolved pokemon, trying to look at the ghost's face. He gasped once he was met by a strange golden eye in the middle of Gengar's forehead that fixated its unnerving glare on the trainer's face. His other eyes were still shut, and the body trembled. "Gengar? Are you okay?"
A deeper voice growled as Gengar shivered, the unblinking eye still glaring into the young man's soul. The ghost moved his arm upwards, out of wherever it had been, and shifted to rest it on his trainer's shoulder. While the touch was initially gentle, the trainer soon felt sharp nails digging into his skin, causing him to yell and recoil in shock.
"Gengar! What the hell!" The human said warily, getting to his feet as Gengar growled again, his eyes slowly opening. All three eyes bore into the trainer, the lower duo wide and bloodshot. The ghost used his long arms to step towards his master, somehow pushing himself forwards despite his limbs being in another plane. A deep, dark chuckle rumbled in the pokemon's throat as his lips curved into a sickening smirk, sharp eyes fixated almost hungrily at his trainer. "Gengar, stop that! This isn't funny!"
The trainer's eyes were wide as he backed off in sync with his advancing pokemon, shaking from panic. This wasn't right. Gengar was a born prankster, but he wouldn't go this far. He quickly reached into his pocket to pull out the ghost's pokeball and fire the recalling beam towards him, only for Gengar to sink into the ground and disappear.
"Gengar? Gengar!" The trainer called out desperately, spinning on his heels while searching for the rogue ghost. He instinctively reached into his pockets for another pokeball, hoping Umbreon's dark type could match the sheer power of mega evolution.
A sudden tug on his legs caused him to scream and fall heavily to the ground, both Gengar and Umbreon's pokeballs escaping his hands and rolling off. His gaze shot over his shoulder, and he let out a terrified screech at the sight of Mega Gengar holding his legs and pulling him down into the shadows with him, that horrible smirk never leaving his face. The trainer frantically thrashed against the ghost's grip, desperately trying to escape from the abomination that his partner had become. His attempts weren't quite strong enough to faze the ghost, who reached his free hand into the trainer's pockets to take out the four remaining pokeballs. "Gengar! Gengar, stop it, it's me! Your trainer! These are your friends!"
To his horror, Gengar only let out an ominous, chilling cackle and stuck his tongue out to entangle the balls and swallow his restrained teammates. The trainer screamed out in raw terror while the ghost laughed maniacly, dragging the young man closer and closer to the void he was sunken into.
Suddenly, a black shape slammed into Gengar, causing him to screech and release the trainer, who took the opportunity to crawl away and shakily stand up, turning around to see the scene. "Umbreon!"
The dark type was furiously thrashing and biting against the mega ghost, growling viciously as Gengar struggled to recompose and grab him. Umbreon was too fast, dancing out of the way and striking dark attacks every time his opponent got close. The trainer noticed, however, that Gengar looked more annoyed than hurt by the barrage of attacks, spewing out retaliatory acid breaths and ghost energy to try and hit the dark fox. He soon started using his dimension travelling powers to evade attacks and get on the offensive side, forcing Umbreon to jump and run around to avoid getting struck.
"Umbreon, watch out!" The trainer screamed, but it was too late: Gengar jumped out right behind Umbreon and slammed a fist coated in ghost energy, hurling the dark fox into the nearest tree. The young man immediately rushed towards his pokemon, eyes widening. "No!"
A toxic sludge fell right before his feet, stopping him on his tracks. Gengar was cackling with murderous glee as he crawled towards the trainer, preparing another shadow attack. The trainer backed off in horror, shivering as he glanced at the fallen fox and back at Gengar. "P-please, Gengar! It's me, damnit! Stop that!"
Right as the ghost was about to fire his attack, Umbreon rammed into him once again, sending Gengar stumbling with a growl. The dark type locked eyes with his trainer and barked, pointing with his snout towards the forest before turning to face the ghost once more, growling. The young man gasped, backing off as his pokemon clashed once more, this time with Gengar noticeably having the upper hand.
He ran. Umbreon had given him one single chance of escaping, and he couldn't waste it. Gengar's pokeball was lost somewhere into the woods, and he wouldn't have time to retrieve it before it was too late.
Gengar hadn't only become more powerful. He had become a monster.
The trainer ran for what seemed like hours trough the dark woods, charging past terrified wild pokemon that might have witnessed the situation. He stumbled and tripped, his back and arms hurting from the minor injuries, but he couldn't stop.
A pained, scared yelp reached his ears after a while, followed by Gengar's terrifying laughter. The trainer felt tears running down his face; Umbreon was defeated. He forced himself to run even faster, even more desperately, for the ghost was coming for him. He wasn't Mega Gengar's trainer anymore, he was prey.
It didn't take long before that maniac cackle started echoing louder and closer to him, the temperature dropping from the presence of the ghost. The young man could feel those piercing red eyes fixated on his back, evaluating him, watching for his vital spots.
A movement beneath him made him look down and scream out in horror. Lying in his shadow, where his head was shaped, was that horrifying yellow eye, glaring up at him. He felt his legs faltering from the effort of trying to turn, and it was enough. Gengar burst out of the shadow with a gleeful snarl, ramming the trainer to the ground with vicious force and letting out another insane laugh as he pinned the human down.
"GENGAR! NO STOP IT, GENGAR! NO NO! NO!" The man shouted desperately, which only made the ghost laugh harder, overjoyed by the trainer's suffering. He felt his energy being drained as Gengar gripped tightly to his head, the other arm firmly pinning him down. The trainer's efforts to fight, to struggle were met by his limbs phasing through the pokemon's body, ineffective. The more he struggled, more intense became the draining. Soon, he collapsed, unable to keep on fighting.
"G-Gengar..." He whimpered, barely able to mumble through the knot on his throat as he wept in fear and despair, watching through terrified eyes as the abomination that had once been his trusted partner smirked in a sickening way and cackled even harder, opening his wide mouth and pulling the young man into that endless void, never to see the light again.
"Gengar's relationships are warped. It has no interest in opponents, unless it perceives them as prey."
-Mega Gengar's Pokedex entry in Pokemon Sun
"It tries to take the lives of anything and anyone. It will even try to curse the trainer that is its master."
-Mega Gengar's Pokedex entry in Pokemon Ultra Sun