King of Souls

The King of Souls is a powerful Sacred Gear, thought to be only in the realms of legend and myth. It is also the central plot point during the King's Throne arc of the story, "Highschool DxD: King's Conquest."

Summary
A Sacred Gear unlike any other, however it's existence is heavily obscured, and due to the lack of users, it has faded into the annals of fiction and myth, and is widely unknown to most. The only ones aware of it are Azazel, who had been lucky enough to see one of it's previous wielders, and Michael, who was there to witness it's creation.

According to the stories surrounding it, it is said that the wielders of the Sacred Gear are generally those who are fated to change the world and all within it, ruling over the order of the world as the "one true king," and whosoever possesses this weapon and unlock it's true potential shall receive God's Legacy, though what this could possibly be is unknown, even to Michael, who rules over Heaven. The only one who seems to be aware of what God's Legacy could be is Amestris Kushrinada, the leader of the Lost Kingdom.

It has been stated various times that the King of Souls' power rivals, if not surpasses, even that of the True Longinus, the strongest Sacred Gear in existence.

Abilities
The King of Souls holds a wide variety of abilities, all stemming from the user's own soul and mind, hence it's namesake. In the hands of Noctis Chulainn Kresnik, it is a variety of weapons, each with their own unique abilities and properties, depending on their form. One such example is when it appears as a greatsword, which holds immense power, able to leave behind a giant scar in it's wake. It is even capable of adopting the properties of other weapons, such as Yuuto Kiba's Sword Birth Balance Breaker: Sword of the Betrayer, which combines the properties of Holy swords and Demonic swords, and Noctis' family weapon Gae Bolg, which is more or less a conduit for gathering magical power, and then amplifying it beyond normal limits.

Forms
The King of Souls is a shapeless Sacred Gear: possessing no physical form, and it's shape is solely dependent on the user. In the hands of Noctis, it appears as mechanical weapons, ranging from swords to spears to axes to greatswords.

Trivia

 * The King of Souls' abilities is derived from the weapon used by Noctis Lucis Caelum, the protagonist of Final Fantasy XV, who's abilities and appearance were inspired to create Noctis Chulainn Kresnik's character.