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. 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.
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 throws it into the ocean and explains that he knew how problematic it was so he didn't intend to use it and that with Tyrion/Khaine's death it was inert. However, this also kind of dooms all of reality to be consumed by chaos. *facepalm*
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.
|The High Elves of Warhammer Fantasy|
|Characters:||Eltharion - Everqueen - Teclis - Tyrion|
|Cadai:||Asuryan - Hoeth - Isha - Kurnous - Lileath - Loec - Mathlann - Vaul|
|Cytharai:|| Addaioth - Anath Raema - Atharti - Drakira|
Eldrazor - Ellinill - Ereth Khial - Estreuth - Hekarti
Hukon - Khaine - Ladrielle - Morai-Heg - Nethu
|Events:||The War of the Beard - The invasion of Naggaroth|
|Misc:||Ulthuan - The Vortex - Waystone - Widowmaker|
|Appearances:||Blood Bowl - Dreadfleet - Mordheim - Warhammer Fantasy Battle|