Hellish cave

James had an important secret mission to carry out: to visit the cave where Voldemort had taken Kreacher and destroy the Horcrux. Before his journey, James bought a locket to exchange with the cup after drinking the potion. One night, Kreacher appeared at Potter Manor. This expedition would be carried out only by him and the house elf. It was dangerous for Regulus to move, although he was not under suspicion.

The journey to the cave took several hours. James needed the elf's guidance. They entered the dark cave, with Kreacher leading the way. Like in the sunken vault, they had to dive through icy waters to reach the other side, where they found a door that required a blood sacrifice, a payment for weakness, to open.

James made a small cut on his index finger, and the door opened when his blood made contact with the surface. Unlike the sunken vault, behind the door in the very cave was a large black lake, so large that James couldn't see the distant shores. The ceiling of the cavern was very high, out of sight.

A greenish, misty light glowed in the distance, seemingly at the center of the lake; it reflected off the perfectly still water below. That was where the Horcrux, or whatever Voldemort wanted to protect, was located.

To reach it, they needed to find the invisible boat that Voldemort had used with Kreacher the last time. The elf had seen everything, so he knew there was an invisible boat.

With this information, James only needed a few minutes to locate the boat. From the depths of the black water emerged the ghostly bow of a small boat, breaking the surface. The boat floated with barely a ripple.

James and Kreacher climbed into the boat, which began to navigate on its own toward the small island in the center of the lake. The darkness was somehow denser than normal darkness. James used his wand as a lantern, but it barely illuminated anything.

'There must be something under the water,' James thought, gazing at the calm, black water.

The potion Kreacher drank made him incredibly thirsty. Voldemort had left, thinking the elf would die, but the potion Kreacher consumed was not fatal. So why did Voldemort leave him alive? It was likely that a normal person would have drunk water from the lake, triggering the other protection Voldemort had placed on the cave to kill explorers.

What the Dark Lord didn't know was that house elves could Disappear regardless of Anti-Disapparation spells. Since Kreacher had fulfilled the mission given by Regulus to follow Voldemort's orders, he immediately returned to his master without drinking any of the lake water. This decision likely saved his life.

"What now?" Kreacher asked, looking at James warily. He didn't want to be forced to drink the potion that left him in a state of pain, desperation, and many other unpleasant things he didn't dare describe.

"You said it caused you immense dehydration, right?" James asked without directly answering.

"Yes. I drank more than ten liters of water thanks to the kindness of Master Regulus," Kreacher replied.

"Hmm, so the purpose of this potion isn't to kill you directly. It's obvious that Voldemort wants you to drink the lake water," James thought, looking at the lake that radiated a very powerful dark energy.

Having already experienced the vaults with their cursed cores, he could tell that this lake emitted a strong dark energy.

James tried to summon water with his wand, but the water disappeared the instant it appeared. He then took out a glass bottle of cold water from his ring. He uncorked the bottle and drank the water.

'This muggle method works,' James thought with a slight smile.

"Take the fake locket. Once you've completely consumed the potion, take the locket and switch it for this one," James said, handing a golden locket to Kreacher.

They would swap the lockets in case Voldemort had set up some security measure to alert him if the locket was missing at the end of the basin.

After handing the locket to Kreacher, James looked at what appeared to be a lamp from some tipi but was a stone basin placed on a pedestal.

He approached the basin and looked down. The basin was filled with an emerald liquid that emitted a phosphorescent glow.

'A potion that will make me experience unbearable pain, relive my worst moments, and dehydrate me almost to death,' James thought, and after a few seconds of mental preparation, he began drinking the emerald liquid.

Within a few breaths, James began to feel the effects of the potion. For a moment, he looked up and saw Peter with a lifeless expression, shouting things at him. He gritted his teeth and continued drinking. As for the physical pain, it was the mildest for him. He had experienced true death; his will was strong.

The hallucinations were the hardest part, but he managed to overcome them. He had experienced Dai Ryusaki's curse. He was used to it, though it was not something he was proud of.

After finishing the liquid, James collapsed onto his sitting, feeling immense pain in his stomach and an enormous thirst. He quickly reached for the glass bottles of water he had taken out earlier and began drinking. Fortunately, the muggle method worked.

Kreacher had already taken care of swapping the lockets. In his wrinkled hand, he held the locket Voldemort so desperately wanted to protect.

James needed several minutes to stop experiencing hallucinations and severe pain that prevented him from standing. He then took out several potions to recover from the physical and mental fatigue.

When James had recovered, Kreacher handed him the locket.

The locket was made of solid gold with a serpentine S embedded in a bright green stone on the front. 'The dark energy emanating from it is incredible…' James thought with a frown. There was no doubt it was a Horcrux. The locket appeared to be a significant relic. The S might belong to Slytherin, but James cared little about the history of the jewel. It needed to be destroyed.

Regulus had researched what could destroy a Horcrux. The options were few. Only two:

The first option: The venom of a basilisk, the most fearsome and deadly creature in the magical world.

The second option: is Fiendfyre, the Cursed Fire. A deadly curse that produces enormous enchanted flames of immense size and heat capable of destroying anything in its path. It is an extremely volatile and sensitive cursed fire.

"Kreacher, return to Regulus. You've completed your mission. Well done," James said, looking at the locket. Kreacher respectfully said his goodbyes and, in the blink of an eye, disappeared.

James boarded the boat and crossed the lake once more, this time alone. He left the boat where it was, and as if by magic, it submerged into the water.

James moved as far away from the lake as he could and placed the Horcrux on the ground. He stepped back about ten meters and drew his wand.

'I can't destroy the cave, or Voldemort will find out that Regulus is a spy,' James thought, taking a deep breath and preparing to cast the Fiendfyre curse.

This curse wasn't as difficult as Protego Diabolica; what was truly challenging was controlling the cursed fire, which could easily turn the caster into ashes if not managed properly.

"Pestis Incendium!" James shouted, making a sweeping motion with his wand.

A powerful jet of infernal fire shot from his wand, quickly forming into large, living creatures. Fiendfyre took the shape of serpents, dragons, and chimeras that twisted and writhed in the air, seeking to consume everything in their path. Fortunately, the cave was large, and its ceiling was out of sight, or James might have caused the cave to collapse in a matter of seconds.

The cursed fire didn't seem to want to head toward the direction of the Horcrux. James, frowning and sweating profusely, raised his left hand, using his telekinetic power to contain the fire, tame the beasts trying to escape, and prevent the flames from consuming the entire cave.

It was one of the toughest battles James had faced in recent times. The ferocity of the Fiendfyre was overwhelming. James managed to direct that ferocity toward the small Horcrux, which was bathed in infernal flames.

The Horcrux began to vibrate intensely under the pressure of the infernal heat. James could feel the Horcrux's resistance, but he knew it wouldn't last much longer. The flames surged over the Horcrux with fury, completely engulfing it. After an agonizing moment, the dark energy of the Horcrux was destroyed. The relic disintegrated into dust, letting out an agonized scream that was lost among the roars of the fire.

Without wasting a second, James pointed his wand at the Fiendfyre, firmly casting the counter-curse. The fiery beasts struggled for a moment, resisting extinction, but eventually began to fade away. The roaring fire slowly extinguished, leaving James alone in the cave, breathing heavily with the relic reduced to ashes at his feet.

'Damn… I want to sleep for days while Gwen hugs me…' James thought, breathing heavily as he sat on the ground and wiped the sweat from his brow.

It took him more than twenty minutes to recover and resume his journey to leave the cave and head back home.