Assassin's dagger

A corpse on the path leading North to Gramps' hut in the swamps has one. Note: This is the only weapon with innate Poison. There is also an animation when you knockdown an opponent. Geralt will then spike his sword into the ground and pull out his dagger and start stabbing him like crazy. Just place this dagger in the bottom storage slot.

Found on Salamandra assassins, among others. Looks like a broad bladed dagger.

Meuchler Dolch Assassin's dagger Dague d'assassin Pugnale dell'assassino Sztylet zabójców