Weapon of Nightmares

Weapon (any melee weapon), very rare (requires attunement)

Any melee weapon can be a weapon of nightmares, but maces, flails, and morningstars are the most common. You gain a +2 bonus to attack and damage rolls made with this magic weapon. An identify spell or an hour of study and focus indicates this is a mace (or other weapon) of disruption.

Curse. This weapon is cursed. Becoming attuned to it extends the curse to you. As long as you remain cursed, you are unwilling to part with the weapon, keeping it on your person at all times. While attuned to this weapon, you have disadvantage on attack rolls made with weapons other than this one.

The first time you use this weapon to kill a creature that has an Intelligence score of 3 or higher, you begin having nightmares and disturbing visions that disrupt your rest. Each time you complete a long rest, you must make a Wisdom saving throw. The DC equals 10 + the total number of creatures with Intelligence 3 or higher that you’ve reduced to 0 hit points with the weapon. If the saving throw fails, you gain no benefits from that long rest (no healing, no Hit Dice regained, no spell slots recovered, no class abilities refreshed, etc.) and you gain one level of exhaustion, but the rest still counts as your one long rest in a 24-hour period.