One of the early experiments of Crassius Bonesworth, this one in particular proved to be a remarkable success, for it showcased the possibility of fusing necromancy and enchanting magic into a potent magical item. The spider forming the hilt of the weapon is kept in a constant near-death state with only necromantic magic keeping it alive, whilst its venomous glands are pierced tip of the blade, thus creating a substanstial flux of fresh venom. It is unknown whether the undead-like spider feels any pain or if it merely a sign of undead bloodthirst, but nonetheless, its appendages are victims to never ending twitching and spasms and its mandibles are trying to gnaw on anything in its vicinity.
Warning: Always try to keep the dagger sheathed or stuck in something, for the spider is still partly alive and succumbs to its bestial predatory nature, even enhanced by the craving for killing common among undead, so it may try to escape and hunt on its own merit, which is a prospect both amusing and horrifying.