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
Sometime after the Magical Golden Flower was removed to save the Queen and her unborn daughter, these mysterious rocks started to sprout out of the ground where the royal guards found the flower. The only thing that is known about the rocks is that they are unbreakable. An ancient scroll from Quirin revealed that the rocks may be caused by the drop of moonlight, with reference of the golden flower which comes from the drop of sunlight.

Role in the Series
Cassandra brings Rapunzel to 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 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 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 it causes the rocks' rapid growth and destroy her tower eventually. 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 encroach on 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, least 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 her unbreakable hair as a drill tip in an effort to drill through the amber, but it fails. In the end, the black rocks converge on Rapunzel when Varian threatens her mother and Cassandra by driving a giant automation. Rapunzel realizes that this was her destiny and 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 possibly made of the same material.

In "Beyond the Corona Walls", Rapunzel tried to activate the power of the black rocks again but it doesn't work anymore. At the end of the episode, Adira showed the remaining part of the ancient scroll, which illustrates a castle called the Dark Kingdom is surrounded by the black rocks.

Trivia

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