In times long before the Oni, the universe was in peril, with no sense of meaning or law. From what we know of Fortinbras, he was born in the confusion of the beginning of Earth. In 'Onimusha 2: Samurai's Destiny', Fortinbras does not make an appearance, but there is an ancient scripture of text that describes how he created the Genma to thrive and do as they please. It is further revealed that he also created the human race, afterwards, as mere fodder for the Genma to feast and enslave upon. At the time Fortinbras was the true ruler of the entire world and universe. However, he seemed more intent on destroying humanity, and did not take pity or remorse amongst the humans. A millenia or so had passed before the Oni came and took pity upon them.
The God of Darkness, who was a fierce Oni God, furious at Fortinbras's leadership, led his race, the Oni, also known as 'the destroyers' against him and the Genma. After another millenia or so, the war eventually reached a climax, with the Oni as the victors. Fortinbras had been destroyed and his spirit was sealed away in a mysterious star, and banished to another universe away from the world. This world that Fortinbras was banished to was the netherworld. This was also the first step of freeing the human race from the Genma. However, the Genma were not completely destroyed and both sides agreed to a truce on the following conditions; 'The Genma would allow the human race to roam free and teach them technology and their own culture to survive, in exchange that a number of humans be sacrificed regularly to the Genma as food and fodder'. This term of peace worked, while the Genma thrived in the netherworld and taught the humans technology and culture to adapt and survive.
In exchange for human sacrifice. Eventually, the humans had learned so much that they began to take matters into their own hands and began to fight back against the Genma. With this term of contract and peace broken, war between the two spilled out all over the land again.
In the first game, Fortinbras minions resurrect a recently killed warlord named Nobunaga Oda, who promises in return to conquer the world and serve as Fortinbras' slave. In his battle with the Saito clan, however, he encounters much difficulty. The demons then kidnap the princess of the Saito clan in order to have Nobunaga drink her blood in a demonic ceremony. It would ensure that Nobunaga would be victorious over the Saito clan. They also kidnap the princess's little brother, in order to kill him in front of her to make her sorrow peak, and make her blood more powerful. The protagonist, a samurai named Samonosuke, and his ninja partner Kaede, save The princess and her brother by killing Fortinbras.
Onimusha: Dawn of DreamsEdit
Fortinbras doesn't appear until the fourth game in the series (although he is mentioned in texts in the other games). Fortinbras is resurrected as his demon form. After he is defeated, he comes back in his human form, appearing as a European man. He is defeated in the end of the game.