Arcane Eater

You gain a +1 bonus to attack and damage rolls made with this magic weapon.

When you hit a mage or magic user with this weapon, they take an extra[roll]1d4[/roll] points of slashing damage. For the purpose of this weapon, "magic user" refers to any creature with the spell casting ability.

Curse. Attuning to this weapon imparts the curse of Nishruu. As long as you remain cursed, you are unwilling to part with the sword, 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. Once per short or long rest roll a d20 on a 1 the Nishruu curse expands around you and attempts to cast Feeblemind DC 17 on you. 

Retribution. If you take spell damage from an enemy you gain advantage on your next attack against that enemy and disadvantage against all other creatures. 

Arcane Consumption. Once per short rest if you have taken more than 25 points of magic damage. You gain magic eater on your next successful melee attack dealing damage to creatures also drains away its ability to cast spells or use spell-like abilities, on hit enemy loses one spell slot of the highest level available. A creature with a spell-like ability that gets hit loses one daily use of its highest-level ability. If this spell-like ability is usable at will, the creature is unable to use it for 1 hour.

You can break the curse in the usual ways. Alternatively, casting banishment on the sword.

 The sword then becomes a +1 weapon with no other properties.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Damage: Additional, Holding this weapon for one hour attunes it to you. Upon attunement you are cursed by the spirit of Nishruu., Damage, Debuff, Combat, Versatile, Sap