Dáinsleif

Dáinsleif (戦乱の剣 (ダインスレーヴ)) is an incredibly dangerous sword kept sheathed by Marian Slingeneyer of GREMLIN, because of its apparent danger Marian only uses it in the most dire of situations.

Etymology
The word Dáinsleif in Old Norse means "Dáinn's Legacy". Dáinn is a dwarf, with his name meaning "Dead" in Old Norse. Taking it as a whole, Dáinsleif means "Dead Legacy".

Concept and Origin
In the Norse text according to Snorri Sturluson in his account of battle known as the Hjaðningavíg, Dainsleif was the sword of the Danish king, Högni.

When Högni draws the sword against Heðinn after the latter offers him compsensation for the abduction of his daughter. Högni states that the sword was made by dwarves and that every time is bared someone must lose his life. He continues that its stroke can never be arrested, and the wounds made from it are never healed.

In Toaru Majutsu no Index it is referenced as one of the signs of Ragnarök, and is referred as a magic sword where humanity could start a war to continue Ragnarök and continue to bring the greatest devastation by swinging it. Although, this may be attested to the drawing of the sword and the battle that ensues after it, Hjaðningavíg, which was a continuous battle that will go forward until the coming of the Twilight of the Gods, Ragnarök.

Appearance
Its true form is never actually shown, and is just referred to as a western-styled sword, which is sheathed inside a golden scabbard. The blade is gold colored, and equipped with a handguard. The weapon apparently had a gold glow.

Principles
The scabbard is as important as the sword itself, as it protects anyone from the instantaneous effects of the blade when it is unsheathed from the sword, as well as part of the overall magic of the sword. Because of its magical nature, the Imagine Breaker can destroy the sword as well as the scabbard.

Scabbard
To use the power of the scabbard, the sword must be sheathed back into the scabbard as part of its ritual. The scabbard is powerful enough to bind Dáinsleif into it, as well as gravity. Here, as long as the target is upon Earth, a clear sphere with a diameter of 3 meters will form at the end of the scabbard, which is the result of the rapid absorption of gravity. The sphere is compressed even further into the size of golf ball, creating an incredibly powerful sphere full of potential energy. Since there are no escape routes, if the wielder wills to create a small hole upon the sphere, an incredibly powerful force would be released from it. Dirt and dust are collected from this absorption of gravity, and when released creates an incredibly sharp and dangerous gray colored blade, similar to Misaka Mikoto's iron sand attack.

By controlling the size of the balloon and the location of the opening, she controlled the size, force, and direction of the giant swords, but that was all she could control. The wielder controlled the hole, so one cannot completely distort the direction taken by the particles emitted from the hole by gravity. As such, when the opening at a time of a release is blocked, then the sphere has a chance of an explosion as the energy has nowhere to go. The blade is a natural phenomenon that is unaffected by Imagine Breaker.

Another effect of the scabbard is to seal other things, such as people into the sphere, like a pocket dimension. The space is dark and lightless like an abyss. The absorption and compression of gravity and its release still occurs during this process, as such, any human still in the sphere will be killed if they are caught when one of effects happen.

Preliminary ability
When used for the first time, where the user raises the sword up, as if an invocation, a flash of lightning strikes from the sky and through the golden sword (apparently not hurting the wielder of the sword), and then with the wielder at the center, a large flash erupts and shockwaves are sent in all directions.

From there, every time the wielder unsheathes the sword even a little bit for the blade to be exposed, according to the legend of the sword, where someone must lose his life every time is bared, as explained by Idol Theory. It is explained that every time the blade is exposed, it emanates fear upon an area. It is unknown at what range the effect has, though since the legend does state that someone must die every time it is bared, then it can be assumed that it is a world affecting spell. The wielder is apparently unaffected by the effect, though this is in doubt if one follows the legend to the letter. The fear is apparently of the unknown magic effect of the blade once it is bared, forcefully shutting down the body, a suicide induced by the unconsciously by the body. It can be summarized as the body killing itself out of fear as it is better to die than to face the bared blade. The effect's target is apparently random.

The affected area of the sword can be negated by the Imagine Breaker, as well as those who are affected by its spell, those who have already "died". The wielder of Imagine Breaker is immune from the effects as well.

Full ability
When fully unsheathed, Dáinsleif is capable of bringing out the powers present at the end of the world by cutting through Phases, allowing the wielder to choose and wield the 'endings' found in the various mythologies. By cutting away certain Phases, which represent the homes of the Gods and the blessings and protection they provide, it releases the calamities that are being restrained by the gods, which manifest from the pure mass of stored power released from the Phase.

Before using the sword in this way, the sword is first used to cut the surrounding space and create a barrier to seal off a portion of the world, preventing the emergence of the calamities from creating effects like that of Angel Fall or possibly breaking the world. After that, the sword is swung and the wielder declares the Phase ("Cut away [Phase] of [Mythology/Religion]") they are cutting through as well as the being they are summoning ("Summon [Name], [Description]").

Beings capable of being summoned by Dáinsleif include:

Baggage City Arc
It is unknown how exactly Marian obtained the sword, but it is implied that she made it herself.

Marian later witnesses Kihara Kagun's death after his battle with Byouri, and hither, she breaks down completely. Disraught beyond any rational measure, Marian takes out Dáinsleif, despite Othinus' pleadings, even lashing out on her. Through its use, Marian downs Saflee Opendays and Oumi Shuri with ease. Kumokawa Maria tries to comprehend what had just occurred, and concludes that it must be the sword, as well as the effects it has on them, stopping their hearts before something even more terrible happens. Marian is impressed by Maria's analysis, althought this will not save Maria. Marian unsheathes the sword, and immediately after, her consciousness begins drifting away from her. Suddenly, a right hand appears from nowhere, somehow allowing Maria to retain her life, and immediately after, she discovers Kamijou Touma appears, finally destroying the distortion that has engulfed Baggage City.

Marian tries to use the swords first effect on Touma, but it had no effect due to his Imagine Breaker. Angry for Touma for denying Bersi's efforts (destroying the distorted world that had filled Baggage City), even then Touma will still fight her. Still with the sword sheathed, Marian tries to cut down Touma, to which he realizes that it cannot be negated by his Imagine Breaker. Immediately later, Touma determines the power of the sheathed sword, which collects vectors in a single point and send them directly out. Marian continues attacking, and sucks Touma in the sphere at the end of the scabbard of the sword, but easily manages to get out, and destroys one of her gold tools. Although this was a set-up that would allow Marian to put Touma in a position in which she could strike her. Thinking quickly however, Touma grabs the fragments of the tools he destroyed to cut his uniform and allow it to be sucked into the sphere of Marian's scabbard. With the sphere collapsing due to the pressure, the sphere explodes, sending Marian back and ensuring Touma's victory.

Still adamant in giving up, Marian threatens in unsheathing completely her sword, but Touma plainly tells her that he knows that she will not unsheath the sword, for she should have done it in the first place, ending all the battles even before they begin. Furthermore, he states that she did not want to draw it, stating that it might destroy more than what she wants destroyed, as well as the fact that she went berserk after Bersi's death, she still managed not to draw the sword. Marian tries, and unsheaths the sword, but could not even do it, when the power of the sword begins to emanate. In her moment of hesitation, Touma steps forward and punches her, defeating her.

Othinus later arrives and after tearing out Touma's right hand off, and crushes the sword in her hands, thinking that it was too dangerous to be used, as it threatened her plans.

Lance of Gungnir Arc
When Marian Slingeneyer catches up to Touma and Othinus in Denmark, she uses the remnants of the failed lance in the form of Draupnir to recreate Dáinsleif. She then uses the unsheathed sword to cut away phases and bring out the embodiments of the end of the world from various mythologies in an attempt to kill her enemies. The sword is destroyed by Touma's Imagine Breaker during the battle.