Black Rocks

The Black Rocks are mysterious sharp rocks that came out of the ground surrounding the spot where the magical flower sprouted from, and later played a part in the return of Rapunzel's long hair.

Background
These mysterious rocks are invasive and unbreakable, and seemingly capable of growing anywhere. They are generated from a strange opal floating in the center of a meteor, the source of the black rocks' power. An ancient scroll from Quirin implies that the rocks, and by extension the opal that creates them, may have been made from a drop of moonlight, with reference of the golden flower that comes from the drop of sunlight.

When the opal came into the possession of a king named Edmund, the rocks grew and ravaged his kingdom into an inhospitable wasteland. With no way to remove the opal without summoning more rocks, Edmund instructed his guard to hide the existence of the opal and his kingdom from the rest of the world.

Unfortunately, the rocks managed to spread to the kingdom of Corona when the Magical Golden Flower was removed to save the Queen and her unborn daughter, and the rocks started to sprout out of the ground where the royal guards found the flower.

Season One
Cassandra brings Rapunzel to the rocks while sneaking her out for some downtime. When Rapunzel reaches out to touch one of them, a powerful energy burst from the rocks knocks the two girls off their feet and somehow grows back Rapunzel's long golden blonde hair.

A few months after Rapunzel touched the rocks, they begin to sprout throughout the land. They start to appear around Varian's village, Old Corona. Every time Rapunzel went near them, her hair would glow along with the rocks, indicating that they are somehow connected. Varian promised to help Rapunzel and Cassandra understand the rocks better and to keep their work a secret. As the number of rocks started to destroy the village, Varian learns that they would not stop "growing", and once they had destroyed his village they would start making their way to Corona. Varian desperately tries to use alchemy in his attempt to stop them, until his experiments get his father crystallized in an amber-like substance.

In "The Quest for Varian", Rapunzel uses her hair to interact the rocks and triggers a shock wave to stun the troops chasing her, but in the process, it causes the rocks' to rapidly grow and destroy her tower. They discover the ancient scroll referencing both the rocks and the golden flower, suggesting the two elements originated from the sun and the moon.

In "Secret of the Sun Drop", the rocks slowly begin to grow closer towards Corona. Following Varian's abduction of the Queen, King Frederic confesses to Rapunzel that he had known all along what the rocks were, revealing that the night of her birth, he was warned by Quirin against removing the flower, revealing he would reap unimaginable consequences and awaken a darkness, which resulted in the black rocks.

During the battle in Old Corona against Varian's army of Automatons, Eugene gets an idea to throw the automatons onto the rocks to destroy them. Meanwhile, Rapunzel and Frederic are trapped by Varian in a failed attempt to ambush him. Varian then uses Rapunzel's unbreakable hair as a drill tip in an effort to drill through the amber to free his imprisoned father, but it fails. In the end, the black rocks completely surround Rapunzel when Varian threatens her mother and Cassandra by driving a giant automation. Rapunzel finally understands that this was her destiny, realizing she shouldn't run away from the rocks, but she is supposed to face them. Rapunzel then grips the rocks and successfully controls them to defeat Varian and his automatons. The rocks then breach the kingdom's border wall and begin to lay flat, pointing to the outside world. Having an epiphany, Frederic admits he thought the rocks' purpose was to destroy Corona but ignoring them only made the danger worse. He encourages Rapunzel to follow the rocks to their destination.

In the aftermath, the black rocks are revealed to be breakable by a mysterious warrior with a black sword made of the same material.

Season Two
In "Beyond the Corona Walls", the episode's prologue reveals that the rocks had existed and corrupted a kingdom twenty-five years ago, before the sun drop flower was uprooted. Adira attempts to cut Rapunzel's hair with her black rock sword but it fails. It is also revealed that the rocks have already spread to other places, among them the city of Vardaros that Rapunzel's group visits in the episode. During the wedding crash, Rapunzel tries to activate the power of the black rocks again but it doesn't work anymore. Adira explains that it's because the rocks had found the Sun Drop, or Rapunzel in this case. At the end of the episode, Adira shows the second part of the ancient scroll, which illustrates a castle called the Dark Kingdom surrounded by the black rocks.

Trivia

 * The black mysterious rocks were inspired by etched Gideon iron meteorites.