The Black Cross King was the father of King Sombra, the son of the King Arthur and Princess Celestia, the founder of dark magic, and the leader of the Black Cross Army in Equestria. The Black Cross Army, also known as the Black Crusaders, are a group of evil ponies that seeks to dominate Equestria and the world. The Black Crusaders swear absolute loyalty to their king and if a Black Crusader even thinks of betrayal the punishment is death. They have secret bases all over the planet, but their main base is in Equestria.
The Black Cross King was first known as Prince Soulander, and he became obsessed with his ambitions to rule over Equestria. But it did not came true since although he had everything his heart desired, he was spoiled, selfish and unkind. Therefore, Celestia and Arthur sought Soulander to have no rights in becoming Equestria's ruler and passed the honor to their future daughter (Princess Shine). In anger, Prince Soulander disagreed King Arthur's ideal on the construction of the Round Stable, a special stable designed to symbolize the equality and fairness of every governor and knight loyal to Arthur. With his aggressive feelings and pure grudge, Soulander created dark magic. Because of that, Soulander became the Black Cross King. He founded the Black Cross Army with the evil creatures he released from Tartarus and began taking over many regions of Equestria. However, when the battle between the Black Cross King and King Arthur came in the Battle of Canterlot, Arthur succeeded in defeating his own brother and sealing him and his army permanently in the gates of Tartarus, but at the cost of the King's life.
However, the legacy of the Black Cross King was passed on to his son, King Sombra. With the ultimate dark magic passed on to him, Sombra promised to continue the takeover of the Black Cross Army in Equestria by taking over the Crystal Empire with his own bare hooves. He has slain the last Crystal Emperor, before he was overthrown by Princesses Celestia and Luna and banished in the arctic north.
Years before Nightmare Moon's return and 1400 years after the banishment of the Black Cross King, Celestia gave birth to another child, named Princess Shine. Though she was born from the Princess of the Sun herself, she was fatherless because Celestia's spouse King Arthur died long before Princess Shine was born. She was given the title "Princess of Color" and the privilege to co-rule Equestria with her mother. Today, little did Princess Shine know that she was the true sister of the Black Cross King who was once Equestria's greatest enemy.
Trivia: The Black Cross King somehow mostly resembles Mordred, a traitorous knight in the English legend who fought King Arthur in the Battle of Camlann and was killed in battle, while mortally wounding his opponent. Also, Mordred was the illegitimate son of Arthur.
Name: Black Cross King
Birth Name: Prince Soulander
Age: 300 (as of 537 C.E.), 1,766 (as of 2003 C.E.)
Birthdate: 237 C.E. (Celestial Era)
Coat color: Silver
Eye color: Red with green
Mane and tail color: No mane (tail is black)
Aura color: Black with purple and green smoke-like mist
Cutie Mark: Unknown
Relations: King Arthur (father)
Princess Celestia (mother)
King Sombra (son)
Princess Shine (young sister)
Princess Luna (aunt)
Princess Cadance (cousin)
Prince Blueblood (cousin)
Twilight Sparkle (cousin-in-law)
Shining Armor (cousin-in-law)
Position: The King of the Black Cross, the Prince of Equestria (formerly)
Occupation: Leader, King, Dictator