Fellblade

In the dim days of Skaven history, the Ratmen waged war against Nagash. To aid in their efforts to destroy this vile Necromancer, they forged this wicked sword, blending Gromril stolen from Karak Azgal with Warpstone. The result of their horrid labours was this long-bladed sword. The weapon exudes power and malice, and those Skaven who use this weapon in battle are almost always consumed by the sword’s hate.

Source

 * Warhammer Fantasy RPG 2nd ED -- Children of the Horned Rat (pg. 83).