This weapon is cursed. A creature that attunes to it is unwilling to part with the weapon, and it cannot break its attunement to it. After taking a soul using this weapon's property, the cursed creature feels the sentience within the weapon and regards it as a trustworthy friend. A cursed creature also feels the need to keep the weapon's sentience a secret. A cursed creature might sneak away to an unoccupied room and consult the scythe for advice, or mutter to the weapon quietly in the heat of battle. If the scythe issues a command or piece of advice to the creature, it must make a DC 15 Wisdom saving throw or follow the advice, ignoring any moral compunctions it would normally have. Otherwise, it only takes it into consideration as if it was a conversation with a normal friend. If ever the creature is dropped unconscious, and isn't a Night Rider, the scythe will slay it and devour its soul.
However, if the scythe determines that the creature attuned to it would be too stubborn or clever to be persuaded, it might take a different approach and completely hide its sentience from the wielder, waiting patiently until it can emerge.
Either way, if the wielder takes 3 souls with this weapon, their worship shifts to Myrkul and the Night Rider within it arises - taking the scythe back and conscripting the creature into the Legion of Myrkul.
Note on Cursed Items: (DMG 138)
Cursed Items: Most methods of identifying items, including the Identify spell, fail to reveal such a curse, although lore might hint at it. A curse should be a surprise to the items user when the curse's effects are revealed.
If it requires attunement, you are not only cursed as long as that attunement exists, but you are prevented from ending the attunement on your own voluntarily.
Remove Curse, or Greater Restoration will end attunement to a cursed item long enough for it to be removed, and the character freed from that curse, but will NOT end the curse on the item.
The only methods to remove a curse on an item typically is the use of a Wish, destroying the item entirely, or in the case of a deliberate curse being cast during creation, the caster must release it.