Black and White Daggers

Weapon (dagger), rare (requires attunement)

These matched daggers are identical except for the stones set in their pommels. One pommel is chalcedony (opaque white), the other is obsidian (opaque black). You gain a +1 bonus to attack and damage rolls with both magic weapons. The bonus increases to +3 when you use the white dagger to attack a monstrosity, and it increases to +3 when you use the black dagger to attack an undead.

When you hit a monstrosity or undead with both daggers in the same turn, that creature takes an extra 1d6 piercing damage from the second attack.

Note: A previously published adventure features a less powerful item with the same name. This is in the wiki as Black and White Daggers (Variant)