As an example, I’ll summarise the story of the Knights of the Fish as collected by Fernan Caballaro in his Cuentos: Oraciones y Adivinas (“Stories of Spells and Curses”).
A hard-working but impoverished cobbler had set out fishing. He fished and he fished until he became so hungry that he thought he might as well take his own life if still he caught nothing. It was just then that he caught a truly magical and beautiful fish. Thanks to its magical powers, the fish was able to tell the poor cobbler that he should go home and cook the fish, give two pieces to his wife, and bury two more pieces of the fish in the garden. This he did.
In the fullness of time, the cobbler’s wife gave birth to twin boys and, in the garden, where he’d buried the two pieces of fish, two plants grew up, each bearing a shield.
When the twin boys had grown up, they each took up one of the shields as knights and decided to make their way in the world. During their travels, they came to a crossroads, where the two young men parted ways.
As he continued his journey alone, the first of the brothers came across a city that was in mourning because every year a young maiden had to be offered up as a sacrifice to a dragon and this year this dreadful fate had fallen to the city’s beautiful princess. Of course, the young man arranged a meeting with the unfortunate princess and he gave here a mirror to use when she encountered the dragon. “Cover this mirror with your veil and hide behind the mirror” he told her “and when the dragon approaches, you must tear off the veil”.
The princess did as she was told and when the dragon attacked, he found himself staring into a mirror at an identical dragon to himself. He roared and he huffed at his identical adversary, smashing the mirror to pieces in the process. But since every fragment of the glass also continued to reflect him, he thought he too and been torn to shreds. This left the dragon distracted and in a state of complete disarray. Seizing his chance, with the dragon so preoccupied, the knight swiftly killed the dragon.
To show his gratitude for so bravely rescuing his daughter, the King of the city gave away the princess in marriage to the heroic young knight.
His young bride, the princess, showed the knight all over the castle and, from one of its windows, he spotted another castle, further away, made all of black marble. “Never go there” warned the princess “for, whoever has been, has never returned”.
So, of course, the knight set out for the black marble castle the very next day!
When he announced his arrival by a blast on his horn and a rattling at the gate, a hideous old hag eventually opened the door. Although strange, ghostly voices cried out to try to warn him off, the knight nevertheless raised his visor and the hideous woman let him in when she saw how handsome he was. In her most cunning and beguiling voice, the wretched hag told the knight that he must marry her – and of course he refused. So, under the pretence of showing him around the castle, the wicked witch killed the knight by dropping him through a trap-door.
At length, the knight’s own twin brother came to the same city and because they looked so alike, he was taken to be his brother. So that he could best help his brother, he did not let on about his true identity, but told the princess that he would have to go back to the black marble castle in search of his lost brother.
Arriving at the dark, forbidding castle, he demanded to know the fate of his brother and the strange ghostly voices found a way of telling him the whole story. He tried forcing the witch to tell him the way down to the trap-door through which she had dropped his brother and when she refused, he stabbed her. At that point, the witch insisted that she still would not tell him the way to the trap-door because she was dying anyway. But she told the knight how he might save her life, with magic plants from her garden. So he went to pick them, gave them to her and was told the way to the trap-door.
Prizing open the door, the knight looked down through it to a cave filled with the bodies of his brother and her many other victims, whom he was able to restore back to life with the same magic plants he had taken from the witches’ garden. He spied yert another cave that was full of the bodies of all the young maidens killed by the dragon, so the knight restored them all back to life also.
At this, the wicked witch was seized with an anger so severe that she died instantly on the spot and, as she fell down to the ground, the black marble castle crumbled into ruins too.