Ancient Moonblade

Shadow Key 
Takes the form of a Moonblade bearing an ancient symbol. 
When wielded you are guided by a fey spirit servant of Balor which harnesses your connection to the Fey and their abilities, making you proficient with this weapon. 
Primary Ability: For a number of times equal to your proficiency bonus per long rest. When making an attack with this blade, you may roll a D100 on the Fey Curse table to bestow a curse on the target. - https://www.dndspeak.com/2017/12/04/100-fey-bargains/ 
Passive Ability: You may speak to the Fey spirit within the blade, Cylnucath, who has seen much battle and recalls every fight in great detail.