Daith's Reaper

The weapon known as Daith's Reaper is a magical artefact, forged by the legendary Daith, from a shard of Khaine's Widowmaker.

History
During the war between the Elven pantheon and the Chaos Gods, Khaine and Slaanesh soon came to blows. Though Khaine was ultimately cast into the mortal realm, he inflicted great harm upon the Dark Prince, and gouged onto his otherwise perfect face a scar that has never fully healed. During that battle, a small shard was struck from Khaine’s Widowmaker, and fell upon the realm later known as Athel Loren. It is this shard, reforged and reshaped, that forms the spine of Daith’s Reaper, a weapon that many Elves hold to be the smith’s finest work.

Source

 * Warhammer Armies: Wood Elves (8th Edition) -- pg. 62