Assassin's Mark (5e Equipment)
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Weapon (dagger), legendary (requires attunement by a Rogue with the Death Strike feature.)
This dagger has a purely copper or bronze-like handle, with a jet-black metal blade of unknown origin. Inscribed onto the pommel of the blade is the phrase "Even the pretender, I strike true," written in Deep Speech.
Certainty. You have a +3 bonus to the attack and damage rolls you make with this magic weapon. When a creature you hit must make a Constitution saving throw against your Death Strike feature, you can add a +3 bonus to the DC. If you do so, the blade loses its +3 bonus to attack and damage rolls until you finish a short or long rest.