Basically, here is how that goes.
Gurdu creates the universe. After he creates the universe Vigoor comes into being as the opposite of it. After a battle between them Vigoor wins, and destroys Gurdu separating him into several different deities. Which destroys the universe at the time.
"In the beginning, a great inconsistency flowed out into the nothing; a nothing that was afraid of its own nothingness."
"...eventually it coalesced into the shape of the first Deity, Gurdu"
"
Gurdu gave birth to all elements, and provided them with order and function."
"In a remote area of space from where Gurdu provided order, there existed the territory of a chaos formed from elements without shape. This chaos, which had been forgotten during the blessings of Gurdu, existed at first as only a instinctual fear, but eventually the chaos began to take shape and a consciousness was formed within it."
"At first this was just a small envious presence, but eventually it grew and became clearer. This marked the appearance of the Hatred."
"The chaotic nature of the Hatred further evolved it, bringing it to the level of a Deity. Thus, the ancestor of all Evil Deities, Vigoor, came into being, and the first conflict was set into motion."
"The ancestor of all Evil Deities, Vigoor, began to devour the territory of Gurdu, beginning at its center.
During endless solar and lunar eclipses, the earth shook and split, the seas dried up only to flood into existence again, and intense winds swept over everything and fires burned down all of existence, ignoring even the concept of time itself."
"Then, a great change occurred. During the throes of the unending destruction, the body of Gurdu himself split into fourths. Each part became a Deity itself, controlling Sentiment, Wisdom, Immorality, and Creation."
"
Eventually they would come together to restore shape to the world again, but in their panic-stricken state after their birth they still had little power."
"In this age of panic, the Serpent, Deity of Creation was the only one of the Deities who managed to keep some semblance of sanity. The Serpent gave birth to a tiny presence, something that no other being would even notice... the seeds of life."
"These seeds managed to take hold and grow in the storm of chaos, and ever so slightly began to evolve, eventually becoming thirteen Dragons, whose destinies were to be found in battle."
"
The Dragons waged an endless battle against Vigoor, who never ceased in his attempts to bring the world into chaos of hatred. Eventually the Dragons were able to banish Vigoor to the very ends of the world."
"However, it was at this time that the youngest and lowest in position of the Dragons, the Dark Dragon, summoned the Hatred with its feelings of inferiority. Due to this betrayal, Vigoor was able to keep from being completely destroyed."
"In ancient times, humans lived in peace with dragons."
"But then came the Dark Dragon, a malevolent giant that wasted no time in plunging the Earth into an abyss of darkness.
Their strength drained by the evil of the Dark Dragon, the remaining dragons placed their last hope in the hands of the humans in the form of one of their own fangs. Thus was born the Dragon Sword, a formidable weapon inhabited by the very essence of the powers of the dragons."
"The bearer of this divine weapon must possess the physical strength, vigorous spirit, and proficiency of technique necessary to wield it properly. If a weaker being were to attempt to use it, the blade may very well feed on the bearer’s soul."
"It was with this sword that the humans were able to defeat the Dark Dragon."
The Serpent who is one of these deities creates the thirteen dragons. The thirteen dragons after a long battle defeat Vigoor, and he separates into the many Archfiends we see in Ninja Gaiden. However, one of the dragons becomes the Dark Dragon. Which allows Vigoor to live on through it. Then in Ninja Gaiden? The Vigoor Emperor uses the Dark Dragon Blade to assimilate the rest of the Archfiends.
Thus becoming Vigoor incarnate. At which point Ryu Hayabusa wielding the Dragon Sword destroys him. The dragon sword which has all the might of the twelve dragons that were not corrupted by Vigoor.
Also, Nioh and Ninja Gaiden exist in the same world. William Adams defeats Jin Hayabusa wielding the Dragon Sword, and he can wield the dragon sword himself with no real side-effects. Only a being as strong as the dragon sword can wield it without their soul being consumed.