Had this idea upon reading the lore and it wouldn't leave me alone. Tell me what you all think.
Blade of the First Emperor
Elven Thinblade (Exotic Weapon; yes, even for elves, the primary school(s) that taught it's use were lost to the First Empire)
Artifact (Identify will not reveal the properties of this weapon; even Detect Magic will not reveal a magical aura. DC 30-39 History check to identify the blade due to wear and tear, Legend Lore works as well.)
Description: The only thing that alludes to the magical nature of the blade is the web of cracks and outright visible fractures that run along it.Those capable of wielding it, may wield it normally despite them.
Little is known about this blade, except that it was crafted using some theories of sympathetic magic that have since been lost with the Cataclysm's destruction of the Empire. Many rumors abounded within those times of legions of Leathaine slaves whose hair was harvested as fuel for the fires of it's forging. Other rumors claim that captured Dwarves were shaved and the humiliation of that act gave the blade power when their hair was used as further fuel for it's forging. There is even a tale of a maiden of the dilynwyr henfyrd who was forced to grieve for years to provide the tears necessary to quench the blade during its forging.
What can be discerned about the blade. however, is that it is a sword of subjugation tied to the very memory and bloodline of the First Emperor of the Manacalon Empire.
Chosen Enemy - As a Standard Action the wielder may designate a target in melee range. Only that target may attack the wielder normally, as all others receive disadvantage on their attack rolls (exception: divine spells). If the wielder attacks any other target than their designated target the effect immediately ends, but is otherwise indefinite (AoE spells do not end the effect so long as the designated target in within said AoE).
Wielder has advantage on all Attacks and Spell Penetration against Dwarves, Halflings, and Elves.
Dwarves, Halflings, and Elves have disadvantage on Saving Throws for spells cast by the wielder.
Rise to the Challenge - The properties as well as bonuses to hit and damage will match those of any magic weapon(s) wielded by the Chosen Enemy.
Note - The wielder receives no bonuses to hit or damage unless conveyed by 'Rise to the Challenge'
Destruction - This weapon will be destroyed (fall apart) if used to attack any elf of Aristocratic/Noble Bloodline*
* - Assuming, of course that the wielder is not an elf Aristocrat/Noble, at which point it is assumed that they bear the blood of the First Emperor and are thus free to play the elvish equivalent to the Game of Thrones freely without risking the destruction of the weapon. Indeed, as they grow in power the weapon itself may yet regain power.