You know what happened between Cain and Abel. Cain, in a fit of selfishness, killed Abel and left him laying in the field they had been cultivating. What we haven't been told is what happened next.
Cain, returning to his home, was confronted by Ilona, Abel's wife of many years. She knew Cain's fits of rage and cowardice only too well, having been married to him before leaving him for Abel. She demanded, "Where is my husband?"
Cain had no answer, so Ilona went to the field and found her dead husband. She took word of this deed to Mnemosyne, future mother of the muses, mistress of Mithras, who had, on a whim, created Cain's parents long ago. Mnemosyne summoned Cain.
"You will take Abel's body to Crete, to Minos. You will ask him to summon his brother, who can restore life to your brother. If Minos refuses, you must kill him."
Cain, knowing the dangers of disobeying Mnemosyne, did as he had been instructed.
Arriving in Crete, he asked for an audience with Minos. After bribing the appropriate officials, he was finally taken, along with his brother's body, to a small temple. Entering by himself, he found it empty. "What is this?" he asked. "What am I to do here?"
The guard who led him to the doorway simply said, "Enter. Do not try to leave." Then he closed the large door, sealing Cain in the small antechamber. As the door closed, all light disappeared and Cain was entirely in the dark.
"Hello?" he said. After some time, he spied a soft light at one end of the room. Walking toward it, he saw a massive throne that had not been there before. Braziers lit themselves on each side, revealing a cavern of seemingly endless dimension. He smelled sulfur. He sensed he was surrounded by small scurrying animals that chirped and moaned softly. Atop the throne, a massive figure stirred.
"Who are you!" said the creature, who appeared to be a very large man with the face of an ox. His lower extremeties appeared to be a nest of very large snakes.
"I am Cain, good king. I am sent here by Mnemosyne, queen to Mithras. I am to ask you to restore my brother to life."
"You are a curious creature to make such a request," rumbled the creature above. "How is it that your brother no longer breathes?"
"Your highness, I murdered him in a fit of rage."
"Why then would you wish him to be restored?"
"My action was wrong, good sir. I would undo it if I were able."
"And so you expect me to do this for you. What would you do for me in exchange?"
"Sir, I have nothing to give you. I have only these clothes I wear and my pitiful body, as you see before you."
"You might give me your life in exchange for that of your brother."
"That I cannot do, good king. I have a wife and children who depend on me."
The creature above suddenly jumped down. He thrust his snout into Cain's face and snorted. "I owe you nothing, human! You are not even of my clan. You deserve death for coming here and asking me for something with no intent to repay me!" His breath stank; Cain shrunk back, near to fainting from the stench.
Then he remembered Mnemosyne's instruction. He took out a dagger hidden in his robe and thrust it into the creature's eye.
Minos thrashed and wailed and twisted in agony and finally dropped to the ground, clearly dead. Cain, who had moved back into the shadows of one corner of the cavern, trembled in fear but watched carefully.
After some minutes, he heard a slithering sound coming from the depths of the cavern to one side. Then he saw it: a snake with a girth larger than Cain's body. It slithered slowly toward the fallen form of Minos until it's mouth was directly over Minos's head. Opening its great mouth, it extended its fangs until one was directly over Minos's mouth. One large drop fell and Minos stirred back to life. The other fang secreted one additional drop, which fell to the stone floor and pooled in a low spot. They seemed to speak briefly in hissing sounds, then Minos returned to his throne. The snake closed its mouth and returned to the depths.
Much later, after sitting silently for hours, Cain felt Minos was asleep. He crept to the pool of serum and scooped as much as possible into his empty water bag. Then he carried his dead brother out of the chamber and returned home.
The cure worked. Abel was restored to life. Ilona rejoiced in his resurrection and returned to Abel's bed and bore him many children.
Cain's wife also bore children. But before Cain could die naturally of old age, as his brother and his brother's children all did, he became sick in his old age. His memory of all things faded; he wandered, delusional and lost until he finally fell and struck his head. This fate, and worse, was unfortunately borne by each of his offspring as well, who passed it to their children. In this way insanity and pain became the legacy for at least half of humanity, who had no idea of the cause, nor how to prevent the onslaught of this deadly heritage.