The swirling water surged up from the depths, rising like a massive, undulating serpent. Percy's heart skipped a beat, the sense of foreboding growing stronger with each passing second. The waves, which had been calm moments ago, now seemed to writhe and twist under the moon's silvery gaze.

Artemis stepped in front of Percy, her silver bow appearing in her hands as if summoned by sheer will. The goddess's usual calm demeanor remained, but Percy could see the tension in her posture—the slight rigidity in her movements, the way her eyes were fixed on the darkened waters. Something powerful was coming, and it wasn't friendly.

"It's her," Artemis said, her voice tight, though she kept her calm exterior. "Nyx has sent something to test us. We must be ready."

Percy's stomach tightened as the sea seemed to churn more violently, waves rising higher, crashing against the shore with brutal force. She could feel the pull of the water, as if the ocean itself was calling to her, demanding her attention, urging her to command it. But the pull of the moon felt equally strong, drawing her inward, into the quiet space where Artemis's presence resonated like a steady pulse.

"Stay close," Artemis repeated, her voice firm. She nocked an arrow into her bow, her eyes scanning the darkened horizon. "Whatever comes, we face it together."

As if in answer, a monstrous shape rose from the depths of the sea. The silhouette of a massive creature, its body long and serpentine, coiled and undulated like an ancient god of the ocean. The creature's skin shimmered with a black, oily sheen, reflecting the moonlight in terrifying flashes.

Percy could hear the whispers of ancient words in the air, like voices of long-forgotten gods calling from the depths. The creature's head broke the surface with a deafening roar, its eyes glowing an eerie red, burning with malevolent intelligence.

"Echidna," Artemis murmured, her voice steady despite the danger. "Nyx's beast of shadows. She has sent her to test us."

"Test us?" Percy repeated, her grip tightening on Riptide. "She's sending a monster to kill us!"

Artemis turned to her, her silver eyes locking onto Percy's with a kind of quiet intensity. "Nyx's tests are never easy, but they are meant to make us stronger. We must defeat this creature, but we cannot underestimate its power. Echidna is not just a monster—she is a servant of chaos."

The serpent-like creature, Echidna, reared back, its colossal body twisting with a terrible grace. With a flick of its tail, the water surged violently, sending a tidal wave toward them. Percy instinctively raised her hand, calling on the power of the sea to form a protective barrier around herself and Artemis. The water responded to her command, swirling in front of them like a shield, deflecting the wave before it could crash down on them.

Echidna hissed in frustration, its gaze locking onto Percy. It knew she was the one to challenge. The serpent's red eyes gleamed with malice, and it coiled its massive body, readying for another attack.

"She's going to keep attacking until we weaken her," Percy said, her voice tense. She felt the moon's power inside her, pulling her toward the light of the sky, balancing the power of the sea that surged through her veins. Artemis's presence beside her only added to the strength she felt.

"We must work together," Artemis said, her voice unwavering. "Use the sea's power to distract her, and I will strike."

Percy nodded, focusing on the ocean. Her hand extended toward the waves as she called to them, bending the water like it was an extension of herself. The sea responded with an eerie hum, and the waves roared, rising higher around them, the water swirling and crashing in sync with her movements.

She could feel the ocean's heartbeat, its rhythm aligning with her own. The power was intoxicating, but also terrifying. It was as if the very essence of the ocean was alive within her, and with a single thought, she could unleash its fury upon the world.

Echidna lunged forward, her massive jaws snapping at Percy, but before she could reach her target, a blast of silver light shot from Artemis's bow. The arrow flew with pinpoint accuracy, striking the beast's side and causing it to hiss in pain, recoiling from the impact. The wound sizzled, and the creature writhed in agony, the shadows of the ocean swirling around it like a storm.

"Focus, Percy!" Artemis shouted, her voice sharp as she prepared another shot. "We need to keep her off balance!"

Percy's focus tightened. She summoned a surge of water beneath Echidna, raising the sea in a massive tidal wave that crashed over the beast, temporarily drowning her form beneath the waves. For a moment, there was nothing but the sound of rushing water, the ocean's fury overwhelming everything.

But then, the beast's glowing red eyes resurfaced, and with a roar, Echidna emerged from the water, more furious than ever. Her body, slick with water, shimmered under the moonlight as she lashed out, her tail sweeping toward Percy and Artemis in a deadly arc.

"Watch out!" Percy shouted, pulling Artemis back with her, but the goddess was faster. She dove to the side, narrowly avoiding the strike as the monstrous tail slammed into the ground, sending shockwaves through the beach.

The battle felt endless. Percy's control over the ocean and Artemis's arrows were the only things keeping Echidna at bay, but each blow from the serpent seemed to get stronger, and their movements were becoming more desperate.

"We have to take her down now," Percy said, her voice grim.

Artemis nodded, her gaze sharpening as she considered their options. "We need to combine our powers—your strength in the sea and my control of the moon."

Percy hesitated for only a moment, then nodded. She extended her arms to the sky, feeling the moonlight pour into her, mingling with the power of the sea inside her. It was as if the forces were aligning, as if she and Artemis were one, their powers merging into something greater.

The ocean roared, and the moon shone brighter than ever before. Percy raised her hands high, calling on the moon and the sea, pulling both powers into a single point of energy. The waters around them surged violently, the moon's silver light guiding them as they came together in one powerful force.

Echidna let out a shriek, realizing too late what was happening. Before she could react, a wave of moonlit water slammed into her, knocking her off balance and sending her crashing into the rocks.

With a final, deafening roar, Echidna fell silent, her body dissolving into shadows as the tide swept her away. The silence that followed was absolute, the calm after the storm settling in like a thick fog.

Percy stood, panting, as she lowered her arms. The power still hummed inside her, the bond between the sea and the moon stronger than ever. She glanced at Artemis, who was still standing, her silver bow lowered but ready.

"We did it," Percy said, though her voice was strained.

Artemis nodded, a flicker of pride in her eyes. "Together, we can face anything."

But as the waves returned to their normal rhythm, Percy couldn't shake the feeling that this was just the beginning. The darkness was still out there, waiting, and Nyx's influence was growing stronger.

And somehow, Percy knew that the true price of victory was yet to come.