The Second Demonic Crusade was a series of attacks carried about by a resurgence of Demons from the planet Hades. Led by the Archdemon Aboleth, these demons wanted revenge against Angels and Humans for destroying their original homeworld; Hell.
The Abduction of Jaya Almis
Jaya Veilsong (at the time called Jaya Almis) was kidnapped by a mysterious doctor who wished to experiment on supernaturals and find the perfect way to negate their abilities. After dispatching a group of heroes to rescue her, including her mother Kittiana Youngblood, the United Earth Government discovered that the doctor was under the influence of a new group of Demons had built a new home called Hades.
The Battle of Oblivion
After failing to rescue her from a secretive space station, the UEG staged an invasion of the planet Oblivion on alongside the Kingdom of Heaven where Jaya was now imprisoned. There her father and mother fought towards the prison, leading to a final confrontation between Kittiana and her rival (and Jaya's captor), the necromancer Lana. Channeling the entity of Zalgo, Lana managed to defeat and kill Kittiana, but Gavin Reeve and Daniel Almis arrived in time to force Lana to flee, allowing them to finally rescue Jaya.
The Battle of Oblivion ended when Captain Dumont of the UEG Navy commanded an orbital strike on the Demon's last stronghold, securing a victory for Heaven.
The Battle of Aumia
In an attempt to regain power against the rest of the galaxy, the Demonic Crusade launched an invasion on the planet of Aumia, intercepting the Zaegrim Inquisition's attempts to take control of the planet. Quickly the United Earth Government and the Alliance of Kings answered calls for help, moving to defend the planet from the Demonic Armies and it's generals; the Horsemen of the Apocalypse. Despite the best efforts of all the leaders and heroes, the Demonic Crusade managed to surround their enemy and secure victory through sheer force.
Formation of the Galactic Systems Union
Deciding that unity was the only way forward, the United Earth Government and the Alliance of Kings met on Mytheria to discuss combining their territories and resources in order to combat the Demonic threat. On that day they came to an agreement to form the Galactic Systems Union, sharing leadership equally between all involved planets via elected representatives on a central council.
At the end of the meeting the celestial Sam'anar appeared, at the time being the ascended form of Daniel Almis, and offered to aid them in defeating the Crusade once and for all. They trusted him, despite his hidden intentions.
Destruction of Hades
Sam'anar transported three heroes; Kittiana Youngblood, Sebastian Duarte, and Elias Bishop to Hades to fight Aboleth. Their goal was to distract the Archdemon while Sam'anar prepared a weapon to defeat him, yet he his true intent was to destroy the planet as a whole with the heroes on it. This plan almost worked. Although Hades was destroyed the heroes were saved by Seh'Makou, the Celestial of Deals, who had tricked Sam'anar into performing the plan in the first place.
Having failed to kill the heroes, who also prevented him from destroying Earth, as per his secret deal with Celestial of Deals, Sam'anar's soul was forfeited to Makou. In exchange for their help Makou let the heroes live, but promised he would return to Kittiana one day for a deal.
With Hades destroyed, the Demonic Crusade was ultimately defeated. Although remnants of demons still remained on the various planets, they never regained enough strength to cause a major problem again. The Galactic Systems Union prospered in their victory, taking in the refugee Angels from the desolated Heaven.