Sword of the Radiant Watcher

Trick weapon (longsword), artifact (requires attunement)
The Sword of the Radiant Watcher is an ancient weapon, meant to contain two entities bound into eternal hatred. One was a hunter and one was a beast, though no one knows which aspect of the blade they were bound to.
Sentience. This sentient weapon has two distinct personalities, each linked to its corresponding transformed state. The Angelic personality (alignment: lawful good) is protective and nurturing, while the Hateful personality (alignment: chaotic evil) is cunning and ruthless. The weapon has an Intelligence of 14, a Wisdom of 12, and a Charisma of 17. It has hearing and darkvision out to a range of 120 feet. The dominant personality may attempt to influence the wielder's actions, particularly when it comes to matters of morality or strategy. However, the sword cannot force its wielder to act against their will. The sword communicates telepathically with its wielder and can speak, read, and understand Common and Deep Speech.
As a bonus action, you can switch between the transformed and untransformed states of this weapon by letting hatred or hope overwhelm you. In both states, the weapon is a longsword. When the weapon's form changes, so too does its dominant personality.
The two personalities within the Sword of the Radiant Watcher are in constant struggle for dominance. When the wielder attempts to switch the weapon’s forms, they must exert their force of will to do so, making a DC 16 Charisma saving throw. On a success, the sword changes form, and on a failure, it stubbornly refuses to do so until the start of your next turn, at which point you can attempt to switch the weapon’s forms again.
Untransformed: Light Form. The Angelic personality is dominant and the sword deals an extra 1d8 radiant damage on a hit.
Curse of Light. When you hit a creature with this sword, you can bind the target to your light (no action required). For the next minute, you are under the effect of the sanctuary spell, but only the bound target must make the saving throw. This effect ends as normal if you take an action that would break it but returns at the start of your next turn.
Retribution of Light. When a target is under the effect of the Curse of Light, as a bonus action, you can bolster up to four creatures of your choice that can hear you within 30 feet for the duration of the curse. While bolstered, they have advantage on their first attack roll against the target each turn, and if the attack hits, it deals extra damage equal to their proficiency bonus of the damage type they dealt. This property of the weapon can be used three times, regaining all expended uses daily at dawn.
Transformed: Shadow Form. The Hateful personality is dominant and the sword deals an extra 1d8 necrotic damage on a hit.
Shadow Chain. When you hit a creature with this sword, you can tether the target to you with a chain of shadowy energy (no action required). While tethered, the creature's speed is reduced by 10 feet, and neither you nor the creature can willingly move or teleport more than 30 feet away from each other. The tether lasts for 1 minute, until you use this property on another creature, if you start your turn more than 30 feet away from the target, or end the tether as a bonus action.
Shadow Consumption. While tethered to a creature with your Shadow Chain, once on each of your turns when you hit the target with this weapon, you can inflict an extra 3d6 necrotic damage and gain temporary hit points equal to the necrotic damage dealt. This property of the weapon can be used three times, regaining all expended uses daily at dawn.
Aborrhant Existence. The sentient entities within the sword are antithetical to each other, and no creature can withstand their presence at once, which is why the blade was sealed in such a way. Once on each of your turns, if both forms of the sword damage the same creature in a single turn, it must succeed on a DC 16 Wisdom saving throw or gain a short-term madness.
In addition, if you deal necrotic damage with the shadow form of sword while the Curse of Light is in effect; or oppositely, if you deal radiant damage with the light form of the sword while the Shadow Chain is in effect, the effect ends and detonates. Each creature within a 10-foot radius of the target must succeed on a DC 16 Constitution saving throw or take 2d6 radiant damage and 2d6 necrotic damage as the antithetical energies resonate and self-destruct. You have advantage on this saving throw.