Had to make some changes to the premise; see author's notes
Two lone figures soundlessly entered the horcrux cave on a rowboat, making a slow and uneasy; almost holy journey to the island at the center of it. Eighteen-year-old Regulus Black (clad in the trailing black robes of a Death Eater) exited the tiny vessel after mooring it, accompanied by his House Elf, Kreacher.
In the recent past, the Dark Lord had asked for a house elf to enter the cave where one of his horcruxes was stored, to test how well the barriers were working. Feeling that further serving his master would be the right thing to do, Regulus had naively volunteered Kreacher for the task. When the elf had Apparated home to Grimmauld Place that evening, he’d told his young master everything he’d seen and witnessed at the cave; which had nearly ended in his dying.
From that point on, Regulus’s eyes had opened to the true nature of the Dark Lord, as well as his many lies. Although the young wizard admitted he was not ready for death, he now only wanted not only to avenge Kreacher, but also to destroy the Dark Lord’s horcruxes as payback. Even if he would die at some point in the process.
Regulus shivered and tried to push that thought to the back of his mind as he and Kreacher made their silent journey across the island. Eventually, inevitably, though, they reached the basin of potion containing the locket at its bottom.
Regulus felt a jerking, tugging sensation on his sleeve and looked down to see Kreacher was pulling at it.
“K-Kreacher has two contradicting orders from you and Mistress Black…..”
Kreacher’s lip trembled as he spoke, and the elf looked so distressed Regulus was surprised he did not burst out into tears where he stood.
“And they are?”
Regulus murmured, impatient to consume the Drink and get this whole business over and done with.
Kreacher shrank back by a few paces; looking uncharacteristically meek, as though waiting for Regulus to strike him after being told the sudden news.
“….M-Mistress Black told me to look out for her son while he was in the Dark Lord’s ranks, and to keep him safe.”
Regulus let out a sigh of exhaustion, sneaking a glance towards the waiting basin.
“Kreacher, this whole business is futile. I should have known Mum would have done something like this- Can’t have her precious youngest in the ground after what happened to poor old Siri, can we?”
Kreacher instinctively came to his Mistress’s defense, as Regulus knew he would.
“Mistress Black only wishes to keep Master Regulus from harm!”
“I know that!”
Regulus snarled, going silent and taking a few moments to weigh the odds. The Drink of Despair, he knew, would drive its victims half-mad anywhere from their second to their final drinks; give them agonizing stomach pains and a terrible, burning need for water. Of course, here they were on an island; surrounded by water!
Which happened to be infested with strategically-placed Inferi who would awaken and come to the surface the second their peaceful waters were disturbed by anyone other than their master, the Dark Lord.
When Regulus first learned what the Dark Lord had done to Kreacher; he’d been rightfully outraged, and had vowed to do something about it. The young man had accepted the duty of destroying the locket horcrux with full knowledge of the fact that as he would begin to drink to sate his horrible, torturous thirst; the Inferi would awaken and drag him into the lake’s icy depths, from which he would never return.
Admittedly, Regulus was not entirely fond of the idea of losing his own life in lieu of destroying the horcrux. But if there was indeed a compromise and a way out as Kreacher was proposing; he was at least willing to hear it.
“Kreacher…. You’re saying that you would pull me out the second I finished drinking, and we’d both Apparate out of the cave as the Inferi noticed my presence?”
“That was the exact plan, Master Regulus, yes.”
Regulus took a few moments to analyze this- as far as the cave’s security went; he had to admit that Voldemort had been clever: Charms had been placed through the entire area to make regular Apparition impossible, but house elf magic was thankfully another matter. If Regulus complied with Kreacher’s compromise; they would Sidealong Apparate out of the cave as soon as the task was done. But there was one more detail…..
“Kreacher, I told you to leave me…. In order for this plan to work, I need you to leave me on the beach after we Apparate out of here- Merlin knows I might not even be conscious by then; but that’s my order. From there, you go home and tell Mum I died- I don’t care how. My original intent was to die since I didn’t think there’d be any alternatives but….. I think that we can do this. And don’t worry about me after you leave me, Kreacher- If the Dark Lord’s ranks knew what we’re about to do they’d start a manhunt the second they heard. If the public becomes convinced I’m dead, I doubt they’ll bother. I’ll start a new life in the Muggle world….. Who would think to look for a dead man?”
Kreacher only nodded, knowingly.
“Is Master Regulus ready?”
Regulus could think of no better time than now.
“Yes, I am.”
Kreacher nodded and went to Regulus’s side as the young man sat down on the damp, liquid-slicked cave floor, legs crossed. The elf tipped Regulus’s head back and forced the potion down his throat. Nothing. It was the same with the next drink, and the one after that; and it was only as Regulus began drinking a fifth time did he begin to scream.
“MERLIN! God….. Oh, someone help….. It hurts…… It fucking hurts!”
Regulus’s anguished howls were raw with agony and primal suffering, and it broke Kreacher’s heart to repeat the process over and over again: Fill the chalice, whisper calm, comforting words to his master and coax him to drink just a little more, a bit more and Master Regulus will be done.
It took roughly half an hour, give or take; before Regulus had sucked in the last sip and the basin had finally been drained empty. It was done.
Regulus meanwhile, felt as though he’d been brutally mauled. His throat was hoarse and dry; like he’d been in a desert for days and days and an oasis was only just out of his reach. There was a sour, twisting feeling deep in his stomach, and the wizard knew it was going to take a miracle to keep him from vomiting. His vision was severely compromised; everything except Kreacher (who was sitting beside him a few inches away) appeared only as darkly-colored blurs in the distance.
Vague half-memories of dark, violent images danced at the back of Regulus’s mind, ready to be unleashed all over again, but he pushed them away and forced himself to stand. In the distance, the young man was aware of Kreacher swapping the lockets; hiding the real horcrux somewhere on his person. But this was now the very least of Regulus’s concerns.
Oh god, he needed to drink; get rid of this damning, torturing thirst…..
Unable to walk without a stagger, Regulus made his way over to the lakeside and knelt, dipping his face into the water like an animal and lapping wildly, slurping and sucking in the icy, delicious-tasting water until his sore throat and aching stomach began to ease.
Just as the first dead, waterlogged fingers began to emerge from the blackened water to claw their way up to Regulus, Kreacher was at his master’s side. Together, they Side-along Apparated with a somewhat-loud pop and vanished from the cavern.
The Muggle beach they arrived was thankfully deserted; quite possibly due to the gray, unusually cool weather conditions for early autumn. Almost instantly, Regulus collapsed on his side on the grayish sand, gasping and struggling for breath.
Kreacher instinctively knelt beside his master, only to be waved dismissively off.
“This….. This must be what Hell feels like, doesn’t it? I must not have drank enough. I’m going mad, I swear I am….. But Kreacher…. You know what to do…. Destroy the locket, tell mum I’ve died. Tell mum, Kreacher!”
Somewhere in the distance, a crow cawed. What appeared to be the gray, sluggish beginnings of thunderclouds drifted idly through the slate-colored sky. For just a moment, almost unnoticeably; Kreacher hesitated. Oh, how he hated to leave his master alone in a place like this, but these were his orders and he could not disobey them.
With a heavy heart, Kreacher took one last look at the master he would never see again before Apparating off to Grimmauld Place, Regulus now unconscious and curled into a shivering ball on the sand.
Regulus did not know how long he lay there; curled into a pathetic fetal position on the cool sand. His Death Eater robes were covered in sand, small stones, and other beach grit; but the young man made no effort to attempt to clean them. What was the point?
His breath was coming in hoarse, ragged gasps now, and Regulus was distantly aware that his stomach was twisting and contracting all over again; and felt the warm, burning sensation of bile scorching up through his throat. Regulus shuddered, convulsed slightly, and heaved onto the grayish sand; vomiting up what appeared to be a mixture of his last two meals and a few sips of the Drink of Despair.
Further down the beach, Regulus could make out the blurred silhouettes of two people: A tall, somewhat heavyset blonde Muggle man and a brunette woman who was probably his wife.
Regulus let out what he hoped came off as a defensive growl when the man approached him, curling further into himself.
“Hey, you all right?”
The wizard whimpered as the Muggle man lightly prodded his back- for some reason even the slightest touch hurt; like every part of his skin and body was unbearably raw.
“…..Don’t bother, Muggle. I’m dead already.”
Regulus slipped his wand from the pocket he’d placed it in and held it close to himself, shivering and preparing to scream as images of demons, murder, torture, and splattering viscera took over his field of vision once again. Hell. Surely he was dead and had gone to hell for the irreversible mistake of becoming a Death Eater.
These were the last things Regulus Black would remember; it turned out, for a long time.