Excalibur

Excalibur is the most powerful of all Holy Swords, formerly wielded by King Arthur Pendragon and now contained within a field of demonic magic within Cocytus.

Summary
The Excalibur is one of the two Holy Swords wielded by The Legendary King, King Arthur Pendragon, of Camelot. According to Rias, the Excalibur is a holy weapon that was created through means of magic and alchemy by those who reached the territory of God. At some point, the original Excalibur was broken, and the pieces were made into seven different swords with the name "Excalibur" in them and two of the blades were each stored by the Catholic Church, Protestant Church of England, and the Eastern Orthodox Church.

In Volume VI of Highschool DxD: Blazing Dusk and Midnight Dawn, Sirzechs reveals to the Occult Research Club that Excalibur was reforged in Heaven and taken to Cocytus in the Underworld for safekeeping. An incredibly powerful field of demonic magic keeps the sword's holy power at bay.

Appearance
Excalibur can change its appearance to whatever the wielder desires, but in its base form, it is a single-edged sword made from some form of blue crystal & blue metal, with runic inscriptions. There is a blue gem in the crossguard and one in the pommel. Its sheath is made of the same metal/crystal combination.

Abilities
Holy Magic: Excalibur acts as an arc of holy magic, especially to devils. It allows the wielder to cast holy spells, even if that is not their affinity.

Destructive Force: Excalibur has the ability to unleash pure, destructive power that can easily shatter anything it comes into contact with, powerful enough to create huge craters.

Mimicry: Excalibur has the ability to transform itself into any weapon and morph into any shape, size, and material its wielder desires. It can even be turned into any regular object. It is even able to create multiple life-like sized copies of a living being.

Speed Enhancement: Excalibur grants the wielder incredibly enhanced speed, fast enough to create after images of oneself. The sword also seems to give the wielder enhanced flexibility, making it easier to avoid attacks at high speed.

Dream/Illusion Manipulation: Excalibur allows the wielder to cast illusions and manipulate dreams.

Invisibility: Excalibur can turn itself and/or the wielder invisible.

Total Control: Excalibur grants the wielder the power to manipulate anything, be it matter, energy or even ether if he or she wishes. With time, the controlled opponent/object will gradually lose their will and eventually become fully subjugated.

Trivia

 * Its appearances are all based off versions of the sword Soul Calibur from the video game series of the same name, Soul Calibur.
 * The base form is α Patrokolos' version from Soul Calibur V.
 * Being, y'know, Excali-fucking-bur itself, the sword is pretty high up there in terms of power. Anyone who wields it is considered to be quite the intimidating opponent.
 * For every action, there is an equal and opposite reaction, right? Same goes for this sword. It has a... twin, so to speak.