Lin En chose the passage just left of center.
The entrance was not very large. A breeze blew out from within, carrying a faint scent of vegetation. Smelling something like that in a cave beneath the desert always felt a little strange.
Ronava did not object. She merely glanced at the passage, then stepped inside. Lin En hurried after her.
The passage was narrow, with many bends. In some places, they had to turn sideways to squeeze through.
The dry gravel underfoot gradually gave way to damp soil, and strange traces began to appear on the rock walls. Green, vine-like plants spread across the stone, giving off a faint glow in the dark.
"There are plants here?"
Lin En said quietly.
"Dendro is abundant around us."
Ronava reached out and touched one of the vines. A faint green light flashed at her fingertip and vanished.
"Dendro from the Ley Lines is seeping out. What this nail is sealing is more complicated than what the previous ones sealed. You should know what lies beneath it."
"Yeah. Apep, the Dendro Dragon."
"So we need to be more careful."
The two continued onward. The passage gradually widened, and the ceiling rose higher. More and more glowing vines spread from the walls to the ceiling, bathing the entire passage in an eerie green light.
The soil underfoot grew softer and softer, and they occasionally stepped on small, root-like things.
The air grew warmer and more humid. Lin En felt as though he had walked into an underground greenhouse.
After about an hour, a broad space appeared at the end of the passage.
It was a natural cavern with a towering ceiling, its walls completely covered in glowing vines.
A shallow pool lay in the center of the cavern. Its water was clear enough to see the bottom, and it shimmered with a faint green light.
In the middle of the pool, the Nail of Retribution rose from the water. Blue light flowed slowly through the runes on its surface.
But a thick, dark green substance floated on the water around the nail, writhing slowly like some kind of liquid and completely enveloping its lower half. The substance churned, contracted, and swelled without stopping, as though trying to seep inside the nail.
Ronava stood at the cavern entrance, her gaze fixed on the dark green substance.
"Apep."
Before she had finished speaking, a deep roar came from the depths of the cavern, sounding as though it had risen from underground. The whole cavern shook.
The dark green substance in the pool abruptly contracted and rose from the water, gathering into a hazy figure.
Its shape resembled a snake, yet also a giant beast formed from intertwined vines. Dark green scales covered its body, with glowing veins of Dendro growing between them and spreading all over it like blood vessels.
It was completely different from the colossal creature Lin En had seen in the game. Perhaps it was a substitute?
"Ruler of Death?"
Apep's voice came from all directions, weary yet tinged with an indistinct hatred.
"What are you doing here? You're not welcome!"
Ronava did not answer. She simply gripped her spear and took two steps forward, placing herself in front of Lin En.
"Lackey of the Heavenly Principles!"
The voice shook several stones loose from the walls.
"A thousand years ago, the Heavenly Principles used this nail to seal me, took my power, and trapped me beneath this mountain."
"I'm not here to help the Heavenly Principles."
Ronava said.
Apep was somewhat shocked to hear that.
"What did you say? Could it be that the Hydro Dragon Sovereign..."
"This nail contains Phanes's power. I'm here to take some of it to deal with Phanes."
Apep fell silent for a few moments, then let out a low laugh.
"Deal with her? You? A lackey of the Heavenly Principles dares say such a thing?"
"Believe it or not."
Ronava was not affected in the slightest. Apep had not fully recovered, making this the right time to negotiate.
"This is a good thing for you."
"A good thing?"
Apep's laughter stopped abruptly, and a chill erupted throughout the cavern.
"Once I absorb the Forbidden Knowledge, I can restore the dragon race and take revenge on the Heavenly Principles. Do you think I'd believe a lackey of the Heavenly Principles now?"
"I won't let you talk like that, you remnant of the dragon race."
Lin En already had his sword in hand and was ready. The negotiations had broken down.
"Fine, very fine! A tiny human like you dares say such a thing!"
The substitute surged up from the pool, dark green Dendro gathering wildly around it. The glowing vines on the cavern walls all blazed with a piercing green light.
The entire space trembled as a great number of rocks fell from the ceiling.
"Lin En, get back."
"No, I'm going to fight this guy with you!"
Apep attacked.
The substitute opened its enormous jaws, and a column of dark green light erupted from deep within its throat, shooting straight at Ronava with corrosive Dendro.
Ronava dodged to the side. Black energy exploded beneath her feet, and she shot toward the substitute like an arrow loosed from a bow.
In a flash, Ronava appeared above it and drove her spear tip straight into a gap between the scales on its neck.
The substitute roared and whipped its body around, flinging Ronava away.
She flipped in the air and landed steadily at the edge of the pool. But the substitute's wound lasted only an instant. Dark green Dendro poured from its veins, completely mending the gash in a few moments.
"Regeneration again?"
"I see it."
Ronava frowned. Like the monster on Tsurumi Island, the substitute clearly possessed tremendous regenerative ability.
The Dendro in the cavern was practically inexhaustible. All the glowing vines throughout the cavern were supplying it with energy.
The substitute lunged again. This time, its body split into several vine-like tentacles that attacked Ronava from different directions at once.
Ronava dodged in the air, thrusting repeatedly with her spear and severing the tentacles one by one. But when the severed tentacles hit the ground, they took root again and quickly grew into new vines, continuing to coil toward her.
"It's using the power of the entire cavern."
Lin En had figured it out. The glowing vines seemed to be the monster's root system. As long as the roots remained, it could regenerate endlessly.
He raced toward the nearest wall and slashed through a glowing vine with his sword. Dark green sap sprayed from the cut, and the green light across the entire wall dimmed for an instant.
The monster hissed in pain, and three tentacles swept toward him at once.
Lin En flipped out of the way and pushed off the ground. A rock wall severed all three tentacles across their middles.
"Well done. Keep cutting the vines. I'll deal with its main body."
Ronava kept flashing around the monster, her spear moving like a shadow and leaving wound after wound on its body, but they soon healed.
Lin En nodded. Anemo exploded beneath his feet, and he sprinted along the edge of the cavern, sweeping his sword through the glowing vines and cutting them apart section by section.
The monster's regeneration visibly slowed. Its roars grew more frequent, and its tentacles no longer attacked as fiercely as before.
Ronava seized the opening, raised her spear above her head, and lifted her hand. Black energy gathered in her palm, growing larger and brighter.
The cavern's green light dimmed beneath the mass of black, and the entire space trembled.
"Go."
The enormous sphere of light flew from her hand.
The blast pierced the monster's head, and black energy exploded inside it. Its dark green scales shattered inch by inch, and Apep let out a deafening howl.
The monster's enormous body crashed heavily to the ground. The glowing vines all went dark at once, plunging the cavern into darkness.
Only the blue runes on the nail continued to glow, their light flowing slowly.
Lin En stood panting among a pile of severed vines, dark green sap still clinging to his sword. Ronava walked over to him, her spear already back in her sleeve.
"Is it over?"
"Yes."
"You do have some strength, but that's all!"
Apep's voice rang out again, and the entire cavern began to tremble.