Black Monster
"My objectives will not be satisfied until I kill every single person who was present as the Moon Child passed through."
The Black Monster was a demon that haunts Endiness. According to legend it appeared 11,000 years ago, exactly the same time the Dragon Campaign ended. It was called a devil that disobeyed the great existence and it was also known for devouring a god.
The Black Monster was directly linked to the Moon Child and the Moon That Never Sets. It was never explained in the legend why the Black Monster devoured the Moon Child, only that it hated the Gods and acted to oppose them.
It was said to be a creature of destruction and fear. The Black Monster was responsible for the death of Princess Louvia and the razing of Neet. It has also be responsible for the same type of event every 108 year since its creation at the end of the Dragon Campaign.
No one knew the true purpose or identity of the Black Monster save one human and a handful of Winglies.
Savior and the Monster
"After counting 108 years, when the Moon that Never Sets glows read, a moon child descends upon the earth, to fill the world with holy bliss." However, what was actually brought was the child of destruction, the Black Monster."
The religion worshiping the Divine Tree, possibly a remnant of the same religion practiced in ancient Vellweb, included the Black Monster in its mythology. The tale of the Moon Child and Black Monster was often incorporated into murals and stained glass windows.
The tale of the Moon Child stated that every 108 years, the Moon That Never Sets would glow red and a Moon Child descended to Endiness. This child was born to bring the world holy bliss. However in every case, the Black Monster descended instead and killed the child. No one knew why the Black Monster came instead of the blessing and what the relationship was between the bringer of destruction and the bringer of salvation.
Truth
"When the Moon Child goes back to the Moon That Never Sets, The Virage Embryo, the God of Destruction will be born."
The truth of the Black Monster that had been lost among the ages or never revealed was that the Black Monster was the savior and the Moon Child was the destroyer.
The Moon Child was the soul of the final species, the Virage Embryo. The Winglies separated its soul from its body in order to prevent the birth of the God of Destruction. This soul, trapped inside the Crystal Sphere, gave the Winglies great power and they abused this power. Absolute power corrupting absolutely.
The Signet Spheres were created to reduce the power of the soul as well as prevent its rebirth. The Moon That Never Sets is actually the body of the Virage Embyro. If the two were reunited, it would herald the destruction of the world. Melbu Frahma in his lust for power created the Divine Moon Objects that held the power to destroy the Signets if he needed more power. While he never used these, they are vital in the Moon Child's quest to return to the moon.
The Virage Embyro and Soa's fate of destruction was supposed to rally all the species as one so that they could defeat the God. The Winglies took the wrong approach to this and suffered the consequences. It would not be until Dart's time that the remaining species would unite to fulfill Soa's true fate for Endiness. Soa did not intend to destroy her world, she wanted her creations to live in harmony and come together in times of crisis for the benefit of the collective.
Identity
" In order to deter the birth of the God of Destruction, she has had to kill the Moon Children. By stopping her own time. She was even called a demon. That is the true self of Rose... the Black Monster!! "
Zieg revealed Rose as the Black Monster. While it was obvious during the game, the party had no clue Rose was this dreadful being and it was difficult for everyone to accept it, especially Dart.
Rose was the final Dragoon alive after the Battle of Kadessa and partially responsible for the release of the Virage Embyro's soul. She learned that the soul would reincarnate as the Moon Child every 108 years and as it grew, the child would use its magical powers to turn everyone it met into its follower. After a long journey, it would reunite with the Moon That Never Sets and become the God of Destruction. In order to prevent this, Rose vowed to kill the child before it could threaten her friends' revolution and the peace the world finally had.
Charle Frahma likely informed Rose of this fate and the two of them worked together to protect Endiness. With her magical power, Charle created a choker that froze time for Rose. She killed the Moon Child over the ages, gaining a reputation as a demon. Her personality and soul hardened as she submitted to having the fate of the world resting on her shoulders.
Rose went from a revolutionary hero to a despised monster in just a couple generations. For 11000 years there were no exceptions. There was only one mistake.
Black Power
"Don't worry, I am carrying out your will. We took this world back together, I won't let them ruin it. Virage... embryo. The moon... with black power."
The Black Monster is a creature of frightening power and ability. It is capable of destroying entire cities. However in the end, the Black Monster is nothing more than a Dragoon. The magic of a Dragoon was likened to Hell on Earth and that magic was used to create the destruction of the Black Monster. Rose simply used her Dragoon Spirit to transform into the Darkness Dragoon. It was in this form that she cloaked herself in darkness and became a being of hate and scorn.
It would have been impossible for anyone to identify the Black Monster as a Dragoon as no one alive would have known what a Dragoon was. Rose had the advantage of being the only Dragoon alive with unrivaled power on Endiness. Only the Winglies could challenge them and they were her allies.