This was also written for Lilybug134's Favourite Era Boot Camp with the prompt "mythical", Owluvr's Character Diversity Boot Camp with the prompt "cooperate", and HedwigBlack's Minor Character Boot Camp with the prompt "bubbles".
At first, he had been proud. His cousin Bellatrix was already a high-ranked officer amidst the Dark Lord's followers and the Dark Lord had approached him in person when he was sixteen because Bella spoke highly of him. He thought of it as a way to restore his family's name after Sirius had wronged them two years prior. He had thought that being a Death Eater was the highest honour one could get and his mind the Dark Lord's idea of rinsing the Wizarding world of Muggles had taken on mythical proportions.
He had been wrong about magnifying the Death Eater involvement. It was scarier en bloodier than he had ever imagined and the first time he had to torture and kill an infant, he had been sick. Bellatrix had scowled at him, told him he was as much an embarrassment to the family as Sirius and Andromeda had been, because Blacks didn't show emotion and they certainly didn't throw up in front of the Dark Lord.
But that was manageable. The thing he had just learned about wasn't.
"Tell me again, Kreacher, what did he do?"
The house elf, stammering and shaking, obediently retold his story. How the Dark Lord had taken him to a cave that was by far the scariest place Kreacher had ever been to. The water was green and the dead had come back to life.
"He had this medallion, and it was evil and whispered. And then, and then the Dark Lord made Kreacher drink this horrible, horrible drink. It was like poison and Kreacher saw…Kreacher saw all the bad things. And the pain…the pain Master Regulus…" The house elf nearly choked on his words. "And he told Kreacher that the medallion would make him immortal as long as it was hidden safe in the bowl. And he put the medallion in the bowl and the poison reappeared and then he left, not looking if Kreacher lived or died. But Master Regulus had told Kreacher to come back, so that's what Kreacher did, come back. It was difficult, but Kreacher did it."
"It's okay Kreacher. No one blames you," Regulus said firmly. "If anything, I blame the Dark Lord. He obviously knew about the poison and took you along for the sole purpose of you drinking it."
Regulus paced through his bedroom in long, angry steps. When the Dark Lord had requested to borrow the house elf, Regulus had been more than happy to cooperate. After all, the Dark Lord could have asked anyone, but he had asked him.
"Immortal, you say? I think that no good can come of that." He stopped dead in his tracks. "Can you describe the medallion to me? In detail?"
"Kreacher can try…" the elf said hesitantly. And he started describing a golden medallion with an engraved S, tessellated with emeralds. Regulus searched around the house for an old necklace of his mother and with Kreacher's description, he managed to transform in a medallion that perhaps was not an exact replica, but it was close enough for what he had in mind. Writing the note took remarkably longer. When you'll find this, I'll be long dead. I want you to know that I have discovered your secret and will destroy the original medallion, in the hope that when you'll finally face your adversary, you will do so as a mortal once again. RAB. He made himself no illusions. He knew he would die in that cave, but that was a sacrifice he was willing to make if it meant he could prevent the Dark Lord from becoming immortal.
"Kreacher, take me to that cave," he ordered.
The house elf looked frightened. "But-but Master Regulus…" he whimpered.
"That's an order."
Kreacher hung his head. "Yes Master Regulus."
Not long after, Regulus found himself on top of cliff that so steep even the birds couldn't nest there. It was cold and the tides of the ocean had him soaked in no time. The light of the moon shining down on the cave gave it somewhat eerie and macabre place.
Kreacher reluctantly showed him how to open the cave and Regulus entered a gauntly place. Tiny bubbles erupted on the surface of the underground lake, leaving a smell of rotten flesh and death in their wake. Involuntarily, he shuddered. This place was evil, he could feel that in every particle of his body.
"Master Regulus, we could get back…" Kreacher pleaded, trying desperately to make his master see his mistake.
"Kreacher, listen to me," Regulus stated. "You are going to bring to the little island in the middle of the lake, I'm going to drink the poison and you switch the medallions once the bowl is empty."
Kreacher's face was horrorstruck. "No, no, Master Regulus can not drink the poison! Let Kreacher do it, Kreacher knows what it does now, Sir.
"No. I need you to get out with the real medallion and destroy it. I do not know what houses inside, but we must prevent the Dark Lord from becoming immortal. So, once you've switched the medallions, you Disapparate out of the cave."
"And Master Regulus?" the elf squeeked.
"You leave me there. If I live to tell the tell, I'll inform anyone. And if I die in the process, well, so be it. "
"But Master Regulus…"
"No, no objections. I have to do this. Even if I beg you to stop, you are not allowed to do so, do you understand? Under no circumstances. "
"Yes Master Regulus."
The house elf obviously didn't agree, but he didn't dare voice this out loud.
"Good. And when I tell you to leave, you leave. No matter what. "
The rest went by in a blur. He knew he must have gotten to the island (by boat? by teleportation?) but when Kreacher had started handing him cups of the greenish liquid, it seemed like his entire world had stopped and had been replaced with pain. Only pain and torture, worse then he had ever inflicted in his life.
"Make it stop!" he screamed. "Stop giving me this poison, you ungrateful house elf, stop it!"
"Master said that Kreacher wasn't allowed to do so," Kreacher said as his trembling hands downed another cup of poison down Regulus' throat.
"NOOOOO!" Regulus shrill shrieks echoed off the wall, bouncing back to him at least ten times reinforced.
"I want it to stop!" Tears were streaming down Regulus' face now. He could not stop them from falling."
"The bowl is-is empty, Master Regulus."
Regulus felt a sort a calmness come over him. He suddenly felt better, the writhing had stopped and he was left with a terrified and damaged soul. Yet somehow, he managed to get the fake medallion out.
"Switch them," he ordered his house elf in a hoarse voice. "Do it now!"
Kreacher did as he was told, but Regulus already had his attention elsewhere. He was just so thirsty…if only he could drink something from the lake…It may not look good, but perhaps the taste wasn't too bad…
"No Master Regulus!" Kreacher shrieked, but it was already to late. The hands of the dead had come from the depths of the lake and pulled him in. He tried to fight it, but they were with too many.
"Get out of here!" he yelled at the frozen elf. "Go away, destroy the medallion!"
The moment his head was pulled under water, Regulus desperately struggled to get some fresh air in his lungs, but the hands of the dead only pulled him under further. Regulus felt his lungs explode and his heart slowly stopping from the cold.
Above the surface of the water, Kreacher watched in horror how his Master disappeared under water and he wasn't able to do anything about it. Master may have been a Black and a brute at times, he had also had moments were he had been kind to Kreacher. With tears in his eyes and the swapped medallion tightly clutched in his right fist, the house elf snapped his fingers and Disapparated out of the cave. He would destroy the medallion, just like Master Regulus had ordered him to do, even if it would cost him forever.