The ancient creature's roar shook the chamber, a sound that chilled the blood of even the bravest champions. Jin gritted his teeth, his golden energy flickering as he fought off waves of smaller monsters that the creature had summoned. The air crackled with tension, thick with the scent of ozone and fear.

Across the battlefield, Nova's energy blade sliced through the air, a beacon of hope in the chaos. "We can't keep this up forever!" he shouted, his usually calm demeanor strained by the relentless assault.

Lira of the Blaze Realm unleashed a torrent of flames, her fiery hair whipping in the inferno she created. "Then we end this quickly!" she retorted, her eyes blazing with determination as she incinerated another group of approaching monsters.

Bjorn, descendant of Thor, brought his mighty hammer down with a thunderous crash. Lightning arced from the weapon, sending shockwaves through the enemy ranks. "For Asgard!" he bellowed, his voice carrying over the din of battle like a war drum.

Amidst the fray, Aria stood guard over the still-unresponsive Eren, her healing light pulsing weakly as she tried to defend against the encroaching horde. "Jin!" she called out, desperation tinging her voice, "Eren's not waking up!"

Jin's heart clenched at the news, but the relentless assault of monsters kept him from rushing to his friend's side. "Just keep him safe!" he shouted back, his fists glowing with golden energy as he smashed through another wave of beasts, each strike a prayer for Eren's safety.

[System Alert: Warning - Round Three critical phase. Survival probability: 18% and falling.]

The ancient creature, towering over the battlefield like a colossus of nightmare, began to gather energy for a massive attack. Dark, crackling power coalesced around its form, the air growing heavy with malevolent intent.

"It's preparing something big!" Nova warned, his keen senses picking up the surge of energy that made the hair on the back of his neck stand on end.

The champions braced themselves as the monster unleashed its devastating blast. A massive beam of dark energy erupted from its maw, hurtling towards them with frightening speed, promising obliteration.

"DEFENSIVE POSITIONS!" Jin roared, abandoning his offensive to reinforce their protective barriers. His voice carried the weight of command, galvanizing his comrades into action.

Nova's energy shield flared to life, a vibrant blue merging with Jin's golden barrier. Lira conjured a wall of white-hot flames that danced and writhed, while Bjorn called down a curtain of lightning that crackled with raw power. Aria, her face etched with concentration, wove a tapestry of light to bolster their defenses, each strand a testament to her will.

The blast struck their combined shields with earth-shattering force. The champions strained against the onslaught, their powers pushed to the limit. Cracks began to appear in their defenses, the dark energy seeping through like tendrils of smoke, seeking to snuff out their resistance.

Just as it seemed their barriers would shatter, fate intervened. The beam suddenly veered off course, as if deflected by an unseen hand. It exploded harmlessly to the side, the resulting shockwave strong enough to stagger the champions but not nearly as devastating as a direct hit would have been.

As the dust settled, a collective gasp rose from the group. There, standing before them where Eren had lain moments ago, was a figure crackling with newfound energy. But something was... off. Terribly off.

A laugh erupted from Eren's lips, cruel and mocking, a sound so at odds with their friend that it sent shivers down their spines. "Finally," not-Eren sneered, his voice carrying an edge that cut through the chaos of battle, "this bastard was weak enough for me to take control of him."

The figure turned, revealing eyes that glowed with an unnatural light, a twisted mirror of Eren's usual warm gaze. "Hahaha! I won't let you take my body," he continued, his tone dripping with sarcasm, "he was full of shit."

Jin stared in shock, his mind reeling as it struggled to comprehend the horror unfolding before him. This was Eren's voice, but the words, the tone – it was all wrong, a perversion of everything his friend stood for.

The other champions exchanged worried glances, the gravity of the situation dawning on them like a cold, creeping dread.

"Eren?" Aria called out hesitantly, taking a step forward, her healing light flickering with uncertainty. "What's happened to you?"

The thing wearing Eren's face whirled to face them, a wicked grin spreading across his features. "Eren's not home right now," it cackled, the sound echoing unnaturally in the chamber, "but I'd be happy to play!"

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Before anyone could react, not-Eren launched himself at the ancient creature with impossible speed, leaving nothing but a blur and a gust of wind in his wake.

"Eren, stop!" Jin shouted, snapping out of his shocked stupor, his voice cracking with desperation. "What are you doing?"

"Has he gone insane?" Lira gasped, her flames flickering wildly, mirroring the chaos in her heart.

"We have to stop him!" Nova yelled, already moving to intercept, his energy blade humming with renewed purpose.

But they were too late. Eren's fist, now wreathed in a dark aura that was a twisted mockery of his usual power, slammed into the monster's hide with a thunderous impact that shook the very foundations of the chamber.

The creature stumbled back, clearly caught off guard by this new development. With terrifying efficiency, Eren's possessed form tore through the monster's defenses, each strike more devastating than the last. Dark energy coursed through his body, distorting the air around him like heat waves rising from scorched earth.

The monster, despite its immense power, found itself overwhelmed by Eren's relentless assault. With a final, bone-crushing blow, Eren's fist tore through the creature's defenses, shattering its core. The beast let out one last defiant roar before collapsing, its massive form crashing to the ground with earth-shaking force.

Eren stood over the fallen monster, his shoulders shaking with barely contained laughter. He inhaled deeply, an unsettling smile spreading across his face. "Ahh, the smell of blood... it's so pleasant," he purred, his voice carrying an edge that made his companions' skin crawl.

[System Alert: Warning - Fourth Trial initiating. Prepare for enhanced difficulty.]

The chamber trembled as new energy began to pulse through its ancient walls. From the shadows, new monsters emerged, their forms twisted and dark. But as they charged toward the possessed Eren, his eyes darted toward them, freezing them in their tracks. The creatures cowered, their previously aggressive stance melting into submissive terror.

[System Alert: Critical Update - Monster entities experiencing fear response. Combat effectiveness reduced by 15%. Anomaly detected in trial parameters.]

In a blur of motion, Eren flash-stepped directly in front of his former companions, appearing so suddenly that even Nova's enhanced reflexes couldn't track his movement. "Can I be a part of this plan too?" he asked mockingly, his voice dripping with false sweetness as he tilted his head at an unnatural angle.

Before anyone could respond, Eren's arm moved in a casual arc. The air itself seemed to split, a blade of pure energy slicing toward his former allies with lethal intent.

"Move!" Nova shouted, barely managing to raise an energy shield as the others scattered. The sword slash carved through Nova's barrier like paper, leaving a deep gouge in the chamber wall behind them.

Lira summoned a wall of flames, but Eren walked through them as if they were nothing more than a warm breeze. "Your powers are impressive," he sneered, "but they're nothing compared to what this body can really do when it's not held back by that weakling."

"Eren, fight it!" Aria pleaded, her healing light pulsing with desperate intensity. "You're stronger than whatever's controlling you!"

The thing wearing Eren's face threw back his head and laughed, the sound echoing off the chamber walls like breaking glass. "Oh, don't worry," he said, his eyes glowing with malevolent glee. "I won't kill any of you – not yet anyway. Instead..." His grin widened impossibly. "I'll make this bastard suffer so much pain that he'll fully give up his control to me. And then, if I'm feeling generous, maybe I'll spare you all."

Jin stepped forward, his golden energy flaring. "We won't let that happen!"

"Let?" Eren's possessed form chuckled, dark energy crackling around him like lightning. "You think you have a choice?"

[System Alert: Fourth Trial initiating in 3... 2... 1...]

The chamber's walls began to pulse with an ominous red light. The ground trembled beneath their feet as ancient mechanisms whirred to life. The cowering monsters in the shadows seemed to multiply, though they kept their distance from Eren's dark aura.

"Perfect timing!" he exclaimed, clapping his hands together like an excited child. "More playmates for us!"

The champions found themselves caught between these new threats and their possessed friend, though the monsters seemed more afraid of Eren than eager to attack.

"We need to contain him," Nova muttered, his energy blade humming with tension. "But these creatures..."

"Split up?" Lira suggested, flames dancing between her fingers. "Some of us handle the monsters while others try to—"

She was cut off as Eren suddenly appeared between them, moving faster than they'd ever seen him move before. "Planning something?" he whispered, his breath cold against their necks. "How about we make this more interesting?"

With a wave of his hand, dark energy erupted from the ground, forcing the champions apart. The possessed Eren stood in the center of the chaos, conducting it like a symphony of destruction.

"The rules are simple," he announced, his voice carrying over the sounds of battle. "Survive the trial, survive me, and maybe – just maybe – you'll get your precious friend back. But fail..." His eyes glowed with sinister promise. "And I'll make sure the last thing Eren sees is his own hands ending all of your lives."

Bjorn roared in defiance, lightning crackling around his hammer. "We've never backed down from a challenge before!"

"True," Eren's dark counterpart agreed, sliding into a fighting stance that was both familiar and wrongly different. "But you've never faced anything like me before."

The champions exchanged quick glances, years of fighting together allowing them to communicate without words. They knew the stakes: save their friend, survive the trial, and somehow manage to do both without getting themselves killed.

"Remember," Jin called out, his voice steady despite the fear in his heart, "that's still Eren in there somewhere. We just need to—"

His words were cut short as Eren launched himself forward, dark energy trailing behind him like a comet's tail. The real battle was about to begin, and none of them knew if they would survive what came next.

To be continued...