Widowmaker

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A weapon, made by Vaul for Khaine, containing immense power. The power to make someone a Mary Sue. It is the Excalibur of the Warhammer world, except it's dark twin that just wants lot of blood.

Warhammer Fantasy

More powerful than any other object in the mortal world. It appears as different kinds of weapons, specifically the type of weapon favored by whoever is gazing upon it and if drawn will keep that shape.

Made for Khaine to wield personally, the Widowmaker somehow wound up in the mortal world at the Altar of Khaine on the Blighted Island north of Ulthuan. When the Warp Gates at the poles of the world collapsed and Bloodletters and Daemonettes fucked over northern Ulthuan, the Everqueen of the time perished. As a result the OG of the High Elves, Aenarion, totally lost his shit. There was a myth that whoever drew the Widowmaker would be imbued with great power, but would be cursed to fail and their entire line would be doomed to want to copycat him. He did it anyway. Notably with everyone from his own gods, to Daemons, to the Chaos Gods themselves, to the ghosts of his dead family all begging him to reconsider and not go through with it. Grimdark. The only other being in the cosmos that was encouraging him to pull the sword was Khaine himself.

It powered him up so much he was able to destroy entire hordes of Daemons. However, it also made him make a string of bad decisions that would result in the beginning of the Dark Elf race. This might have been Khaine's original plan, since it ended up with the Dark Elves coming into being, and as a result the two groups of elves have fought since. During the last great battle against Chaos in the first invasion, Aenarion used the Widowmaker to kill the avatars of all 4 Chaos gods and countless Daemons. He was mortally wounded, so he mounted on his dragon and flew across Ulthuan to return the Widowmaker to it's altar. There he took off his chestplate, and stood shirtless and bleeding to death for a moment. He then vanished, body and all.

High Elves have spent a great deal of time and lives keeping the Dark Elves off the Blighted Isle, and once there try to disrupt the rituals needed to unlock the Widowmaker to be drawn by a Dark Elf. So far, they've been unsuccessful. They've also wasted enough resources to take over the world ten times over trying to get it. Blame Malekith. Which is weird, given that in his novel Malekith had had the chance to take up the Widowmaker and decided against it realising it was an obvious Trap in a demonstration of character growth and having learned from his father's mistakes.

In The End Times, Tyrion succumbs to madness and draws Widowmaker for himself, becoming the Avatar of Khaine and the leader of the Aestyrion. Widowmaker is given a pretty nasty statline; it ignores armor, wounds automatically, has the Multiple Wounds (D6) property, and it can't be destroyed by enemy spells or by magic items (pretty nasty!? This thing hits harder than freaking Ghal Maraz and the Slayer of Kings - fitting given it was made by a literal god of blacksmithing)

Lore-wise eventually Tyrion as Khaine's avatar is defeated by a combination of Malekith and Alith Anar. Malekith, to the horror of everyone else present, takes up the Widowmaker. While everyone's soiling themselves at the idea of Phoenix King Malekith with the sword of Khaine, he threw it into the ocean and explained that he knew its power was a trap and with Tyrion/Khaine's death it was rendered powerless.

The Widowmaker also appears in Total War: Warhammer 2, and can be drawn by any of the Elf factions after securing the Shrine of Khaine settlement in Nagarythe and upgrading it to the Shrine of the Widowmaker, though other factions can take it after killing/wounding its wielder in battle. Keeping the weapon will cause penalties to public order, diplomatic relations and eventually worsen to increase army upkeep while making the user reduce allied leadership around them & the public order of their local province, but who gives a fuck about that when it will also cause fear against high elves and wood elves and allow the wielder to plow an utter rapetrain though the enemy by murderlating anything in their way. Also note that the game doesn't called it "widowmaker", but "sword of khaine".

40k Version

In the lead up to the War in Heaven Vaul tried to buy the freedom of Isha and Kurnous with 100 mighty blades. Khaine agreed, but Vaul was unable to complete the blades. He snuck a mortal blade in and tried to pass it off as the real thing. It...didn't end well. Fans theorize this mortal blade was the Widowmaker. There's also some debate as to whether or not the blade that Farsight found was left by the Eldar. If so, it's possible the supernatural nature of the sword is of a divine nature.