Cursed Fang

Weapon (dagger), rare (requires attunement)

This crooked, jet black dagger curses the blood of any creature hit with it. You gain a +1 bonus to attack and damage rolls made with this magic weapon. When you hit a creature with this weapon that isn't a construct, plant, or undead, that creature is cursed until the end of your next turn. While cursed by the dagger, it has disadvantage on Dexterity (Stealth) checks made to hide and must make a DC 15 Constitution saving throw whenever it magically regains hit points. On a failed save, the creature instead takes necrotic damage equal to the amount healed (maximum 20).

There's no antivenom for a bite like this. This tooth brings only sickness, for which there is no escape.