Runeblade
Weapon (greatsword), legendary (requires attunement)
A runeblade is a weapon of pure evil that serves as both a weapon of mass destruction and a repository for a lich lord’s soul. Each runeblade is unique, possessing a design that is reflective of the lich lord’s personality and aesthetics.
You gain a +3 bonus to attack rolls with the weapon, and creatures cannot have resistance or immunity to damage dealt by an attack made with the runeblade. It has the following additional properties.
Curse. The runeblade is cursed. When you first become attuned to the runeblade or finish a long rest while attuned to the runeblade, you must succeed on a DC 25 Wisdom saving throw. On a failure, you are compelled to commit a single evil act within the next 24 hours that results in the death of an innocent. If you fail to accomplish this task, the next time you hold the runeblade it consumes your soul, killing you instantly and preventing you from being restored to life by all but the most powerful magics.
Lifebreaker. Once per turn when you make a weapon attack with the runeblade against a living creature, you can choose to unleash a 90-foot cone of cleaving force. Each living creature in the area is targeted by your attack in addition to the original target, taking its damage on a successful hit.
Loyalty. You cannot be disarmed of the runeblade unless your hand is removed in the process, nor can you be compelled to relinquish possession of the runeblade against your will.
Personality. The runeblade has the personality of the lich lord to whom it belongs. While each lich lord is unique, the process of becoming such a creature invariably twists one’s soul toward evil. This makes the runeblade a malevolent and vile companion, always seeking to free itself from its confinement and once again menace the world.
Sentience. If a lich lord is slain, its soul is transferred to the runeblade and transforms it into a sentient weapon with the lich lord’s Intelligence, Wisdom, and Charisma. The runeblade can perceive everything you can perceive, otherwise it has hearing out to a range of 120 feet.
The weapon is in effect the lich lord, containing all of the lich’s experience and knowledge. While in this state, the runeblade can communicate telepathically with you while you are attuned to it. If a non-evil creature is attuned to the runeblade, the lich lord’s Rejuvenation feature is suppressed while so attuned.
Ref: TPKB1 p125