Here’s the unpublished short story where I developed the idea for the film:

(Based on actual events.)

Pope Innocent VIII glowered from his bed at the alchemist, who was tied and spread-eagled across a wooden rack in the center of the papal bedchamber.
The hemp ropes and stained spars of the rack seemed raw amidst the tapestries and portraits that decorated the walls. The pope and the alchemist were a similar contrast. Innocent had a bald doughy face with the curled lip of an indignant toad while the alchemist was all bone and gristle, with a nose sharp as an owl’s beak.
At a nod from Innocent, the monks standing beside the rack rotated the cross bars, tightening the ropes. Screams echoed through the chamber, gradually withered. “My wife, my children,” the alchemist croaked. “You burned them alive… Why have you spared me?”
“An excellent question,” said Innocent, “You were indeed to be burned, but I heard a whisper, a rumor, that through your dark art you discovered the secret of life everlasting.”
“Even if I knew that secret, I would tell you nothing,” the alchemist groaned.
Innocent motioned to the monks, who gave the rack a second twist, a third. The alchemist’s left hip popped like a bottle uncorked. Screams. More screams.
Innocent nodded at the monks, but before they obeyed, the alchemist cried: “No more! l will reveal… everything.”
Innocent pulled himself up, rolled to the bed’s edge, and with vast effort stood, his legs spindling beneath his bulk. He waddled forward, close enough to smell the alchemist’s sour breath. “The secret?”
The alchemist’s reddened eyes narrowed to glare so intense that Innocent stepped back a pace. “Spare my life,” said the alchemist, “I will make you immortal.”
“Excellent!” Innocent clapped his hands in delight and triumph.
Three days later, the alchemist limped into bedchamber and bowed to the bedridden pope. “Are you ready to begin?”
“Yes, and more than ready,” Innocent said in a petulant tone. “You’ve turned the entire place upside down!”
The rack was gone, replaced by a copper brazier glowing red beneath an iron pot of boiling water. Above the pot, wreathed in smoke and steam, hung a coil of copper tubing in the midst of which was a miniature bellows. On the ends of the tube were tapered needles of polished brass.
“Old age results from the aging of the blood,” continued the alchemist. “And therefore youth and immortality lies in its constant replenishment. These needles, and the warming device beneath the tube, encourage and augment the flow of blood from one body to the other.”
Innocent eyed the needles. “Yes, yes, so you’ve explained, several times. Get on with it!”
“Just as you say.” The alchemist walked to the door, addressed the monks waiting outside: “You may admit the benefactor.”
A young man was thrust inside the room. He wore green felt, orange hose (torn at one knee), and tattered pigskin slippers, a street bravo who’d seen better days. He gave the alchemist a companionable half-nod and then, seeing and recognizing the Pope, fell to his knees in terror. “Scusami, scusami,” he repeated.
The alchemist put his hand upon the youth’s shoulder and raised him to a chair beside the pope’s bed. Heartened, the youth, still glancing sidelong at the Pope, whispered to the Alchemist. “Where is the gold that you promised me?”
The alchemist turned to Innocent. “A ducat, please, Your Holiness.”
“A ducat!” cried Innocent, outraged.
The alchemist shrugged. “Is a gold coin too dear a price for the gift of life?”
Grumbling to himself, Innocent pulled a velvet purse from beneath his pillow, extracted a gold coin and handed it to the youth, who clutched it tight.
The alchemist rolled up the youth’s sleeve, wrapped a bit of cloth around the upper arm, waited a few heartbeats, took up the needle at one end of the tube. Holding the youth’s arm steady against the wall, the alchemist jabbed it into the youth’s arm who cried out, bit his lip. “Think of the gold,” the alchemist suggested.
The alchemist brought the opposite needle to the pope’s bed. Innocent eyed it with suspicion. “Will it hurt much?” he asked.
“Only for a while,” said the alchemist as he bound and tightened a silk rope around Innocent’s arm. He ran the needle along Innocent’s ivory wrinkles, dowsing for the perfect spot, then lunged the needle. Innocent yelped and glared.
“The worst is over, Your Holiness.” The alchemist knelt and fetched from beneath the bed a sloshing wooden bucket, from which he extracted a wriggling slug. “In order for the treatment to efficacious, we must extract your worn and tainted blood.” He raised the pope’s nightgown, laid a leech upon a vein-marbled leg. The creature inched forward, bit down, drank. The alchemist dipped into the bucket, again and again, until the pope’s limbs were a mass of translucent, writhing slime.
Innocent, who had been barber-bled before, made no complaint.
The alchemist reached up to the bellows, began a series of slow and regular pumps. Innocent lay back. “If this doesn’t work,” he said, brows darkening, “I’ll burn you alive.”
“It will work, Your Holiness.”
From time to time, but the alchemist released the bellows and tapped the tube, which rang like a dulled bell. “We must not allow coagulation,” he explained.
Pump, pump. Tap. Pump, pump.
Innocent felt his mind clear, limbs lighten, breath deepen.
Pump, pump. Tap. Pump, pump.
The candles seemed brighter, the air almost sweet.
Presently, the alchemist ceased to work the bellows and began to detach the leeches from Innocent’s legs. “How do you feel?” he asked.
“Extraordinary!” the pope declared. “Wonderful!” Then he noticed that the youth lay slumped and motionless in the chair. “What is this?” he demanded.
“He gave his life that you might live,” said the alchemist, as pried the ducat from the dead fist and placed it in his own pouch.
It had been years, nay, decades, since Innocent had felt so robust. He held court for the first time in months, summoning the cardinals from their mistresses to attend him. Despite their cries of joy at his recovery, many, he observed, seemed dismayed rather than pleased. “The vultures!” he thought, “I’ll make them pay!”
Innocent spent a pleasant day in giving commands, settling scores, eating well, drinking deep.
The only drawback was the dreams. Each night his slumber was plagued with endless copulation. Sporting a penis the size of a Soppressata sausage, his dream-self coupled with a vast array of objects, animals, and people, and he awoke exhausted each morning, his bedchamber tainted with the stench of stale vinegar.
Innocent summoned the alchemist, who listened with calm sympathy. “I was expecting something of the sort, Your Holiness. The sudden influx of youthful blood has created an imbalance of humors. Fortunately, the solution is simple: you need more blood but of a more choleric temperament.”
Innocent nodded. “Make it so.”
The second benefactor’s face was ruddy, almost purple, with whiskers like mold growing on a rotting apple. When offered the coin was offered, he snatched it like a famished dog and made no resistance as the alchemist prepared his veins.
If blood were wine, the first treatment was a tart Chianti. The second was more like port wine, lush and sweet. Even before the donor died, Innocent felt as if he were sparkling with vigor. So vivid was this sensation that Innocent barely noticed when the alchemist pried the second ducat from the corpse’s hand.
Ah, what a change! Even after a long day stuffed with meetings, rituals, and correspondence, he felt well enough to totter around the palace grounds, followed a stream of courtiers and servants, all of terrified he might stumble and blame them for an injury.
The nightmares, however, did not subside, but were even more distressing: night hags, oozing wounds, severed hands that crab-crawled up his chest, and worms that sang a piping chorus as they popped in and out of rancid flesh. He woke in the morning bathed in salt sweat.
He summoned the alchemist, who was once again apologetic. “Patience! Another imbalance of humors has occurred. To achieve your optimal condition, you will require a final treatment.”
“Yet another?”
“As with the Trinity, three is the number of perfection, Your Holiness.”
“It had better work,” Innocent growled.
The third benefactor was slight of build, skin as pale as a turbot’s belly, eyes as round as a marmoset’s.
“Are you certain you’ve selected correctly?” Innocent asked. “This one looks as if he’s been living in a tomb.”
“He is a scholar, Your Holiness,” said the alchemist, as if this answered all objections.
“A scholar, eh?” Innocent eyed the specimen with suspicion. “Where did you find him?”
The alchemist’s grinned. “There is no shortage of penniless scholars, Your Holiness.”
“Ah.” Innocent handed a third ducat to the alchemist, who gave it thence it to the scholar, who sat down, staring somewhat wistfully at the golden disk in his palm.
So intent was Innocent upon savoring this infusion, that he barely felt the needle’s prick. But as he felt the first thrill of new blood, Innocent noticed that the scholar had already collapsed into the chair.
It was too soon. Something was very wrong. Innocent tried to raise his hand to signal for the alchemist to stop, but his limbs would not move.
Pump, pump. Tap. Pump, pump.
The alchemist spoke: “I gave this man a dose of hemlock before I brought him into the chamber. The numbness you feel is the poison in his blood.”
Innocent cried for help but all that emerged was an airless bleat.
Pump, pump. Tap. Pump, pump.
The alchemist continued: “You may perhaps be wondering I am killing you as an act of vengeance. Permit me to ease your mind. Your death not my revenge.”
Innocent watched and fumed in silence.
The alchemist released the bellows, touched the scholar’s neck. “He is dead and you will soon join him.” The alchemist plucked the coin from the corpse’s hand, reached into his pouch, pulled out the previous two. “These coins were to be my payment but since the final result of my physic has not perhaps been entirely to your satisfaction, I will return them to you… in due time.”
Innocent, immobile, could only glare.
“It will not be long now, Your Holiness. Now, if I might be so bold to suggest that this would be an excellent time for you to reflect upon your afterlife.”
The suggestion shocked Innocent out of his impotent rage. It had not occurred to him before—but it did now—that being murdered in this fashion made him a martyr and, as such, he would earn the most honored place in Heaven, at the right hand of God.
The alchemist interrupted this pleasant thought. “Do not deceive yourself. The youths whose blood we drained were my companions in your dungeon. The first was a rapist who preyed on street urchins; few in the peddler’s quarter escaped his lust, they say. The second was a highwayman, who slew his victims—dozens of them—lest they recall his face. The last was a heretic who bowed down before Satan, naming him ‘Lord.’ All three died unconfessed, pouring their sins into your body. You are thus condemned to Hell, not once but threefold. This is my vengeance.”
As the room darkened, Innocent had a vision of his skin slashed, his bones crushed, his flesh burnt, mingled with the penetrations and abominations of his nightmares, but with no waking, stretching eon upon eon, an everlasting orgy of anguish and woe. He sensed rather than felt the alchemist place a golden coin upon each eye and thrust the third onto his flaccid tongue.