Black Rocks

The Black Rocks are mysterious sharp rocks that came out of the ground surrounding the spot where the magical flower originated from, and later played a part in the return of Rapunzel's long hair. The rocks are later revealed to be the destructive creations of the Moonstone used for the purpose of defending itself, but also longing to be reunited with it's counterpart the Sun-Drop.

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. Anyone who came close to the opal would meet an unfortunate end. 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 began 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 with 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 on 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.

In "Lost and Found", the spirit of Lord Demanitus, inhabiting the body of Vigor the Visionary, reveals to Rapunzel, Eugene and Pascal that long ago, an ancient power fell from the heavens and divided itself into the Sun Drop and the Moonstone, both of which seek to reunite themselves. After reassembling the Graphtic Scroll, Demanitus urges Rapunzel to use the incantation in the scroll to unite the Sun Drop and Moonstone, saying the ancient power can either be used for good or evil, and warns her that if she doesn't go through it with, the black rocks will reawaken and seek her out, destroying everything in their path.

In "Destinies Collide", the origins behind the Moonstone and it's connection to the Dark Kingdom is revealed. The Moonstone came from a single drop of moonlight that fell from the heavens and had the ability to destroy any who would seek it's power and claim it for their own. Centuries passed and the power of the Moonstone continued to grow and spread and from it's power also grew the Dark Kingdom. The Dark Kings, rulers of the land took it upon themselves to keep the Moonstone and its power at bay and to make sure it would never fall into the wrong hands and the legacy continued on, generation after generation. King Edmund assumed the legacy of his ancestors to ensure the Moonstone's safety, but had enough of the damage and destruction caused by the opal's power and tried to destroy it. However, his attempts failed and caused a catastrophic event that almost lead to the destruction of the Dark Kingdom, but also the loss of his arm and the death of his wife. In the aftermath, King Edmund ordered his subjects to leave and never return for their own safety, including sending away Eugene, his only child and heir to the throne. King Edmund's final order to the Brotherhood, consisting of his most trusted and skilled warriors, Quirin, Adira and Hector, was for them to stop anyone from seeking the Moonstone. With everyone gone, King Edmund remained behind in the Dark Kingdom to continue his family's task and legacy of further ensuring the opal be kept hidden away from the world and to stop anyone seeking it for themselves.

The Brotherhood went their separate ways to ensure the Moonstone's existence be kept a secret, determined to uphold King Edmund's last wish. However, Adira believed the best way to stop the Moonstone's power was to seek out it's counterpart the Sun-Drop. After spending years searching, Adira found the Sun-Drop in Rapunzel who harnessed it's power and saw her as the key to restore balance to the world by finally uniting the Sun-Drop and Moonstone together. Adira explains the reason she has kept her true intentions a secret is because she is unaware of what will happen to Rapunzel when she connects with the Moonstone. Despite the possible risks, Rapunzel and the group continue onward and Rapunzel's hair begins to glow and react as they get closer to the Moonstone, but King Edmund is determined to keep Rapunzel away. Edmund convinces Eugene to aid him in destroying it, believing his return is a sign it's his destiny to destroy the opal for good after all the damage and destruction it's caused throughout it's existence. Eugene is reluctant, but agrees to help his father after learning of his mother's death and the possibility the same fate might befall Rapunzel. However, after facing great guilt and conflict, Eugene has a change of heart and fully devotes himself to helping Rapunzel and later manages to convince his father that Rapunzel has come to free him from his supposed destiny.

Afterwards, Rapunzel enters the chamber, where the rocks react to the presence of the Sundrop, pointing directly to the Moonstone. A path is created for Rapunzel to cross and as she approaches, the Moonstone reacts to her presence and removes itself from its cage. Rapunzel reaches out to the stone, prepared to fulfil her destiny. Before Rapunzel can touch it, however, Cassandra suddenly thrusts her arm forward and takes it herself. Horrified, Rapunzel asks Cassandra what she's doing and as the Moonstone begins to glow, Cassandra angrily declares that she is fulfilling her destiny. She raises the Moonstone and its raw power is great enough to send Rapunzel, Eugene, and Pascal flying backwards across the bridge. The Moonstone begins to radiate with undiluted, raw power as Cassandra struggles to control it. The walls in the chamber begin to go haywire as Cassandra finally takes control of the stone, driving it into her chest as she screams. The Black Rocks begin to sprout at an unstoppable pace, covering the bridge, the platform, and the area near the doorway. The stone begins to drastically change Cassandra's appearance, completely destroying her armor and replaces it with a dark blue and black outfit made from the rocks themselves. The energy of the Moonstone courses through her and envelopes her to the point that her hair and eyes change to bright blue. Rapunzel, Eugene, and Pascal look on in horror, witnessing Cassandra transforming from friend to foe as well as the the fulfilment of Demanitus' prophecy.

Trivia

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