Moonstone Opal

The Moonstone Opal is a supremely powerful, ancient opal gem that had descended to Earth from a single drop of pure moonlight, thousands of years ago, along with its heavenly counterpart of healing and light: the golden Sun-Drop Flower.

Lore
Millennia ago a drop of moonlight fell from heavens and became a strange floating opal, known as the Moonstone, when it landed on earth with a drop of sunlight that bloomed into the Sun Drop flower the moment it landed in a place that is extremely far away from where the Moonstone had landed on earth. The opal being of a darker, life destroying force while the flower is a life giving force of power, the Moonstone began to create sharp, large black, blue glowing, invasive and unbreakable rocks that countlessly grow out of the ground as a means of defence, whenever someone, who isn't the holder of the Sun Drop, tries to clam its power for themselves. Because of the destruction that the Moonstone and its black rocks create, a kingdom was built on the land that the Moonstone opal had landed on from the heavens, so the Dark Kingdom's rulers of monarchs and warriors could protect the world from its harnessed power of death and destruction.

When Lord Demanitus heard about a mysterious event that had led to an ancient power to fall from the heavens and divided into the Sun Drop and the Moonstone, both of which seek to reunite, he and his three students began to seek them out for research proposes. Even thuogh Demanitus was able to place his findings in a scroll that he calls the Graphtic Scroll, his students betrayed him when they summoned his archnemesis, Zhan Tiri, as he too seeks the ancient power that can either be used for good or evil. After he defeated the warlock and came to the concussion of how valuable as well as dangerous his research is if it were to fall into the wrong hands, Demanitus divided the scroll into four pieces before scattering them. In hopes that they'll be found by the right hands that would use the two divided powers for good, as two of them were able to fall into the hands of two people that are aware of the Moonstone's life-killing and destructive power.

During the rain of the last Dark King of the Dark Kingdom, King Edmund, he grew tried of the damage caused by the Moonstone and tried to destroy it once and for all, where his rash actions nearly destroyed the Dark Kingdom in a catastrophic event that killed the queen. Riddled with guilt and determined to keep his surviving subjects safe, Edmund ordered for a kingdom-wide evacuation. Including his infant son, Eugene, while Edmund's most trusted warriors (Quirin, Adira, and Hector) formed a group known as the Brotherhood in order to keep all away from the Moonstone. Edmund, however, remained in the Dark Kingdom to keep the stone's magic out of the wrong hands.

After Sun Drop flower was removed from the ground so it could be used to make a cure that will save the Queen and her unborn daughter, however, the Moonstone began to send out its black rocks to it's original location. As they spread from the Dark Kingdom to the kingdom of Corona, where they began to sprout out of the grounded spot that the royal guards had found the flower. Quirin had tried to warn King Frederic of the danger that would be unleashed upon Corona by the removal of the flower, with a piece of the Graphtic Scroll, he had uncovered before coming to Corona, but Frederic had choice to ignore his friend's waring so he could save his family.

Eighteen years after the black rocks came to Corona, Cassandra, brought Rapunzel to them while sneaking her out for some downtime. When Rapunzel reached out to touch one of them the moment they stared to glow a bright blue color, a powerful energy burst from the rocks knocked the two girls off their feet and somehow caused Rapunzel's long, golden hair to grow back. Before the rocks began to "chase" them, but had manged to get away from after they destroyed a stone bridge.

Afterwards, the rocks began to sprout throughout the land, where they started to appear around Varian's village, Old Corona. Every time Rapunzel went near them, her hair would glow along with the rocks, revealing to them that they are somehow connected to her. Varian promised to help Rapunzel and Cassandra understand the rocks better and to keep their work a secret, as King Frederic didn't want anyone to speak about them. As the number of rocks kept on growing in larger numbers and started to destroy the village, Varian knew right there and then that they would not stop "growing" and once they had destroyed his village they would start making their way towards Corona. Varian desperately tried to use alchemy in his attempt stop them, until his experiments got his father "crystalized".

When Rapunzel and her friends got word from Varian, in "The Quest for Varian", and went to Old Corona where they find Quirin's piece of the scroll, Rapunzel used her hair to interact the rocks and triggered a shock wave to stun the troops chasing her, but it caused the rocks' rapid growth and destroyed the tower eventually. By the time they were able to properly look at the ancient scroll, it revealed to them that while the Magical Golden Magic came from a drop of sunlight, a drop of moonlight may have been the cause of the rocks.

On Rapunzel's nineteenth birthday, during the events of "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 reap unimaginable consequences and awaken a darkness, which resulted in the black rocks.

During the battle in Old Corona against Varian's automatons, Eugene gets the idea to throw the automatons onto the rocks to destroy them. Meanwhile, Rapunzel and Frederic get captured by Varian in a failed attempt to ambush him. Varian then uses her unbreakable hair as a drill tip in an effort to drill throw the amber but it fails. In the end, the black rocks converge on Rapunzel when Varian threatens her mother and Cassandra. Rapunzel realizes that this was her destiny, she wasn’t supposed to run from the rocks, she was supposed to face them. Her hair then touches them, causing the rocks to destroy all of Varian's automatons. This whole time, Frederic thought there purpose was to destroy Corona, but ignoring them only made the danger worse. The rocks then breach the kingdom's border wall and began to lay flat, pointing in a certain direction, which Rapunzel is encouraged to follow. In the aftermath, the black rocks are revealed to be breakable by a mysterious warrior with a black sword made of the same material.

In "Beyond the Corona Walls", Adira attempted to cut Rapunzel's hair with her black rock sword but it failed. In the wedding crash, Rapunzel tried 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. At the end of the episode, Adira shows the remaining 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 of the Sun Drop and Moonstone's origins, how they seek to reunit themselves with each other and the work he had one on them. 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 that if she does not got through it with, the black rocks will reawaken and seek her out, destroying everything in their path.

Role in the Crossover
Since the opal was created from a drop of moonlight, much like how the Golden Flower came from a drop of sunlight, it could mean that their magic may come from MiM. While their destructive side and dark color - when they are not glowing blue - and the life-killing power of the Moonstone that creates them could be a result of Pitch Black getting a hold of a small part of MiM's power and uses it to creature a weapon of sharp, spreading desertion, or to just bend the rocks, or just the opal, to his command. A bit like how he altered Sandy's dreamsand to create both his Nightmares and the black Nightmaresand.

Which could lead MiM to using the flower's connection with the opal and its rocks to give Rapunzel the ability to connect with them, in hopes that she'll be able to break Pitch's hold on them. MiM could even give Hiccup the knowledge to forge a sword, like the one Adira wields, or any other kind of weapon that can break them, should there come a time where they may have to defend themselves from them.