Two millenniums ago, there was a war. Between the human world and the other... the Underworld.
But somebody from the Underworld woke up to justice, and stood up against this legion, alone. His name was Sparda. Later, he quietly reigned the human world, and continued to preserve harmony, until his death. He became a legend.
The Legendary Dark Knight, Sparda.
Around 3000 years ago, Mundus has arisen to power, proclaiming himself as the sole ruler of the Demon World, after usurping the throne of the previous Demon King, Lucifer. Mundus wasn't alone, as he was assisted by his own lieutenant and blood-brother, Sparda. Sparda would eventually become the general of the Demon World for sometimes during the next 1000 years of Mundus' rule.
As the Demon World general, Sparda was given a privilege by Mundus to infiltrate the Human World, where he would eventually interacting with many humans there. In the same time, Sparda had already taught the demon twins, Baul and Modeus and the two eventually become Sparda's apprentice. Sparda, too, was accompanied with a demon scholar named Phineas, being appointed as Sparda's master strategist as well. Sparda was also assisted by a demon blacksmith named Machiavelli as well.
However, as time flies, around 2000 years ago from now, Sparda, who had seen the humanity inside, saw the real tyranny and injustice from his own brother, Mundus. Sparda would eventually rebelled against Mundus, but before it happened, Sparda had made a pact to both Baul and Modeus to live true to their aspirations and to carry them out.
After a long struggle, Sparda was able to defeat Mundus, weakened him for awhile and sealing him into Limbo, which was one of the Nine Circles of Hell, and for this reason, Mundus' army eventually retreated to the Demon World. To ensure that none of the Mundus' army wouldn't escaped to the Demon World ever, Sparda used his own blood, alongside the blood of the human priestess (Lady's ancestor), in conjunction with his sword and his magical amulets. He poured this demonic energy into his beloved sword (Devil Arm Sparda), and to close the door between realms, the Temen-ni-gru, though this also robbed him of the lion's share of his abilities. As an added measure, he also imprisoned within the tower its gatekeepers - Cerberus, Agni & Rudra, Nevan, Beowulf and Leviathan - and took the names of the Seven Sins in order to imprison them and set them as seals keeping the Temen-ni-gru underground.
Following his victory against Mundus, Sparda, with the help of a trickster demon named Kongzu (who was the father of Eclisfula) would eventually sealed the Hell Gateway in Fortuna using Yamato, and the souls of the fallen Fortuna warriors were kept sealed inside the sword Fuhuo, which was later given to Kongzu many years later, when Kongzu tried to blend with humans as the member of the Order of the Sword. It was said that for the next centuries, Sparda became a benevolent ruler of Fortuna, as well as the protector of Vie de Marli prior to his disappearance. The Legendary Dark Knight, that was the name that sticked into Sparda.
During his time in the Human World, Sparda realized that his power started to weaken as time passed. Machiavelli would eventually built a coffin-like machine in order to temporary restore Sparda's power for awhile.
Sometimes between the late 20th century and the beginning of 21st century, Sparda decided to make a deal with Kongzu (who now took an alias Canto Hung) for one more time Their mission was to investigate the presence of the revival of Alastor, once the executioner for both Lucifer and Mundus, and also Sparda's mentor. Their mission was indeed quite successful, as they sealed Alastor inside his own Devil Sword, Alastor. After that, they went separate ways (Kongzu would eventually sold Alastor and Ifrit to his associate, Enzo Ferino). Some times later, Sparda, who appeared in his human form, eventually fell in love with Eva, a human woman. Their happy marriage were gifted with twin sons, Dante and Vergil, and this new family would eventually resided in a mansion named Paradise, located not far between Limbo City and Red Grave City. Sparda remained there for a few years, until his so-called death for unknown circumstances. Sparda's absence would definitely meant Mundus would be able to breach into Paradise, and Mundus eventually sent his demon hordes there. Eva, who wasn't able to locate Vergil, hid Dante inside of a closet and told him to run if she never returned. She ultimately gave up her life in order to protect her sons, as she was murdered by Mundus himself.
Despite Vergil and Dante believed that Sparda was already dead even before Mundus killed Eva, the truth wasn't exactly what it seemed to be. In reality, before Mundus' attack on Paradise, Mundus already tricked Sparda before (and before this, Mundus already killed Machiavelli, and Sparda's coffin machine was destroyed by Mundus as well) for his crime against his own race millennia ago. Sparda, the Legendary Dark Knight, was eventually captured by the Demons, banished into the Demon World, and was condemned to endure eternal agony as a prisoner of Mundus. Yet, Sparda never came back.
AUTHOR'S NOTES:
Sparda's demon form is highly inspired by this design by CocoaLegend on Reddit! This guy has actually deleted the original post, but in case you missed it, here you go!