Ones some hundred years ago, when both Thor and Loki were very young, basically stupid teenagers wanting to proof their manhood. They had been going to Midgard on occasion, just to hang out, have fun and be trouble makers.
Neither of them really put much thought into their actions nor what their actions may bring, to them, both of them, it was all good for a laugh.
At one point Thor had seen a big beautiful stag and declared that stag was going to be his dinner! It would make a good meal for a god. He easily crushed its skull with his hammer and then slung it over his shoulder as he and Loki walked to a nearby house where a midgardian lived.
Thor had laughed and presented them with his big kill, telling them to roast the deer so they could all feast together that night.
The people who lived in the house though, had been horrified! It was a father living with his son and daughter, his wife and their mother had passed away not long ago. As it turned out, that stag was special to them. When the stag had been a foal, their late mother had found it with a broken leg and taken care of it, raised it as her loyal pet. Now they considered it all there was left of her.
When the stag had become an adult, it had come and gone, it was part of their own family, it had even saved the father from wolfs! They owed that stag their fathers life. And they were furious.
The family demanded compensation, nothing less than gold would be able to make up for it! Gold enough to cover the entire stag!
Thor and Loki had looked at each other, then finally agreed Thor was going to stay and Loki would go fetch the gold.
Loki though was not about to go ask their father for such a stupid allowance and give away their misdeed, so instead he went to one of the kings leaving on Midgard and simply stole a sack of his gold.
It didn't take long for him to return and easily covered the entire stag with the gold.
The familys eyes which beforehand had been stricken with grief were now lightning up in pure astonishment and then pure greed! So great was this greed, that there was no room what so ever for sorrow and they just let Thor and Loki go.
"Oh by the way!" Loki had exclaimed from the door. "You might not want to keep that gold, it's cursed." he stated. "Who-ever owns it will face great misfortune, not just you people, but your descendants or just who-ever steals it. Anyone who has it will suffer a pretty horrible death."
Thor gave Loki an odd look, but Loki just smirked and the two were on their way.
About a hundred years after, Loki and Thor had all but forgotten about this incident, that is until they suddenly stumbled upon the exact same pile of gold Loki had originally handed over and a man standing above it.
"NO! This treasure must be destroyed! Cause it is cursed! It has brought nothing but death and ruin to my family! I SHALL THROW IT INTO THE RIVER!"
That made Thor and Loki look astounded at each other and Loki started to ask around, then finally they got the story.
After Loki and Thor had left, the father of the house had declared the treasure for himself. His son had not been happy about this and demanded he got his share but his father refused.
That had lead to his brother to kill his father and claim the gold for himself.
Then the daughter in the house, his sister had demanded her share of the gold, but her brother had refused, saying he didn't become a father slayer to share his gold and had taken all the gold to settle on a mountain top where he wouldn't allow anyone to take it.
Enraged by this, the sister had found herself a strong man to marry and soon after gave birth to a son. She had raised this son with stories of the treasure which rightfully belonged to him, and finally the son had become an adult and went on a quest to get revenge on his uncle who demanded all the gold for himself, still sitting up there on the mountain simply because he refused to share.
The sisters son did indeed go up there, and found a man whom had become frail from refusing to leave his treasure. The kill was easy and the son returned in triumph with the treasure.
A young woman in town then started to bate her eyelashes at the son, and suggested they should marry.
The son was very happy about this as he had always had warm feelings for this woman, it was just first only now she started to notice him.
The son had assumed it was because she finally recognized his bravery, the truth though was different, she did not love him, only his new found wealth!
Soon she carried this son a child, which was also a healthy boy, she did though get extremely protective as her husband started to talk about sharing their treasure to better their community, refusing to ones more become poor she killed her own husband and allowed herself a life in luxury where she was cruel to all of her servants, until they finally revolted and killed her leaving her own child behind, her son.
The man standing right in front of Thor and Loki now, the great grandson of the man they had originally given the gold to in compensation for the killed stag! Declaring the treasure should be disposed of.
By this realization both Thor and Loki was wide-eyed looking at the young man, Thor eyes became shadowed with worry and he turned to Loki. "Loki.." he spoke in a pretty frightened voice. "Maybe… you shouldn't have handed mortals a cursed treasure."
At that Loki burst out laughing. "HAHAHAHAHA! Thor! What are you talking about?!"
"The treasure is cursed!" Thor exclaimed. "You said so yourself!"
"Are you really that stupid Thor?" Loki asked. "Of course it isn't cursed, I just made that up on the fly! There are no such things as cursed treasures you idiot!"
Gobsmacked Thor looked at Loki.
"It's human greed! That's all this is!" Loki exclaimed in a humorous voice. "Greed and stupidity." he pointed at his own head.
Thor just kept look just as astounded and just as stupid as Loki was holding his own stomach as he howled in laughter.
He had barely done anything himself! The humans had done it all to themselves!
It had been a valuable lesson back then, often all you needed to do was to send the ball rolling and allow everybody else to do the dirty work.
Already now Loki saw… Hydra agents falling like flies as they were accusing each other, just as predicted. Their paranoia growing as all he had to do was giving the occasional poke.
And their mistakes made other people aware… Nick Fury started to become aware, that something was going on.
The house of cards, so carefully build… And Loki was witnessing it fall. Oh humans… so predictable.
This was going to be fun!
AN:
The story in this chapter is a re-interpretation of the Norse Myth "Fafners treasure." which is indeed about a cursed treasure originally given by Loki as a compensation for accidentally killing the familys son. It is though Odin whom accidentally kills the son.
In the story though, Loki steals the treasure from a dwarf and it is indeed cursed, and it then brings death and ruin to the family for generations until the last son in the family throws it back into the river where Loki originally stole it and killed himself to keep the location of the treasure a secret.
So yeah major changes is, Odin killed the son but Loki goes get the treasure for him. The dwarf he steals the treasure from warns Loki that the treasure is cursed and anyone whom owns it will be met with ruin. Loki doesn't care since he isn't going to keep it anyway.
He warns the family about it though, but they are so overcome with greed they don't care.
It is the brother who goes to the mountain with the treasure who is called "Fafner." and he actually transforms himself into a dragon to guard this treasure (So there's your origin for that trope.) The battle he has with his not nephew but grand nephewn (Grandchild of his sister) for the treasure is rather epic and involves help from Odin.
Then there are multiple stories about the different generations and how the treasure ruins them, until again, the last person in the family line, a man called Sigmund throws it back into the same river where the river dwarf live and Loki first got it, and kills himself so no one knows where the treasure is.