Greater Dagger of Venom (3.5e Equipment)
From D&D Wiki
Greater Dagger of Venom: This jagged, straight edged +3 dagger allows the user to use a poison effect on a target that has been hit by the dagger (as the spell,save DC 20) as a free action. The wielder must activate the poison ability right after the attack hits. The wielder can decide whether to use the effect or not and can use it up to 3 times per day.
Weak necromancy;CL 8; Craft Magic Weapons and Armor, poison; Cost 10,500 gp + 750 XP; Weight: 1 lb.; Market Price: 18,000