Dragoon
As the titular being, Dragoons played a large role in the game. Not only were Dragoons responsible for freeing Humans from Wingly subjugation 11,000 years ago, the entire playable cast was able to transform into these beings and with this power they took on the God of Destruction.
origin
"This is a tool to amplify the insanity."
The Darkness Dragoon Spirit was originally created before or during the Dragon Campaign. A Dragoon Spirit was the pure essence of a dragon's soul at the last moment of life. Rose was the first and only person to be recognized by this spirit. Dragoons were often called the incarnations of dragons, as they possessed power that the original dragon used.
Rose, with the guidance of Zieg, fought and killed a Darkness Dragon to receive its Dragoon Spirit. Rose had carried the spirit ever since and it had never been recognized by anyone else. Before Shirley handed over the White-Silver Dragoon Spirit to Dart, she mentioned that it came from 'a' White-Silver Dragon rather than 'the' White-Silver Dragon. It's possible that Darkness Dragon was a type of dragon rather than a particular dragon.
A Dragoon Spirit's source of power was the insanity humans made themselves enter in order to fight. However it was possible to be overwhelmed by this power. Only strong willed individuals could hope to control dragons. Dragoon Spirits could also only be used to their full capacity during conflict. It was difficult for a Dragoon to transform outside of battle. The spirit granted the Dragoon the ability to fly, cast magic and control dragons under its influence. It was also possible for a Dragoon to have a vassal dragon accompany them.
Dragoon Spirits attracted one another and Dragoons will gather as a result. The first instance of this was during the Dragon Campaign. Afterwards the spirits vanished along with the Dragoons. The spirits only gather at times of great need and disappear as soon as they are finished their task. Dragoons possessed the strength to take on gods and are the greatest weapon of humans.
rose's awakening
"In order to liberate Humans, we have to sacrifice you, Dragon. Now is the time to decide, extinction or war. "
Rose received the Darkness Dragoon Spirit when she killed a Darkness Dragon. She was assisted by Zieg who likely told her that killing a dragon would yield a spirit just like his Red-Eye Dragoon Spirit. Rose explained to the dragon that Humans were going to fight back against the Winglies and it was the dragon's choice to either lie down and die or fight back. The dragon accepted Rose's choice of war and a new dragon was created from its remains.
This vassal dragon was named the Black Burst Dragon Michael and it fought along side Rose during the Dragon Campaign. Michael was the strongest dragon and apparently the only vassal dragon that survived the war. Rose continued to use the powers of her Dragoon Spirit to protect the revolution her friends fought for. It is this spirit that allows Rose to take on the persona of the Black Monster as well as allows her to fight foes far stronger than a normal human could ever hope to fight. Rose remains living in order to maintain the peace she fought for so long ago.
powers
"The Darkness Dragon is the strongest Dragon of all..."
The Darkness Dragoon Spirit came from the strongest of dragons (excluding the Divine Dragon) and its powers were as formidable as its source. The Spirit had both offensive and healing spells, though the healing spell caused damage to an enemy in order to heal. Offensively the spirit granted great strength to its user.
The Dragoon Spirit had no 'evil' connections however due to its dark element, many of its spells could be seen to have evil cogitations. The Spirit could open a gate to Hell, a gate that sucked in all enemies and sent them to its namesake. The spirit could also create and destroy pocket dimensions as well as attack astral forms.
Rose did not have any fondness for devildom or demons and did not see them as allies in any sense. However the Darkness Dragoon Spirit may have had a connection to the dead or those it had directly killed.