Terminus Est

This is the end

Terminus Est is described as being "Light to raise, weighty to descend." This is due to (in addition to the obvious ethical quandary, in that raising the sword causes no harm, but the descending stroke ends a life by decapitation) a channel in the spine of the blade, containing liquid hydrargyrum, another name for mercury.
Terminus Est is described as having been one of the last works of a famous swordsmith—so in addition to its inherent utility, it is also an antique; a shopkeeper once described it as being worth ten times the value of her shop and its inventory.
The sword is double-edged, having both a "male" and "female" side, designed to sever only male heads with one edge and only female heads with the other.
Terminus Est was designed (originally) for executions. As such, the tip of the blade is squared off, and cannot be used for thrusting.

Homebrew

Terminus Est

Weapon Heavy, Two-Handed Very Rare (this item requires attunement)

You gain a +3 bonus to Attack and Damage Rolls made with this Magic Weapon. In addition, the weapon ignores Resistance to slashing damage.
When you Attack a creature that has at least one head with this weapon and roll a 20 on the Attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without The Lost head. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, has legendary Actions, or the DM decides that the creature is too big for its head to be cut off with this weapon. Such a creature instead takes an extra 6d8 slashing damage from the hit.
The first time you Attack with the sword on each of your turns, you can transfer some or all of the sword's bonus to your Armor Class, instead of using the bonus on any attacks that turn. For example, you could reduce the bonus to your Attack and Damage Rolls to +1 and gain a +2 bonus to AC. The adjusted bonuses remain in effect until the start of your next turn, although you must hold the sword to gain a bonus to AC from it.

Type	Damage	Damage	Range	Properties
Martial Melee	2d6+3	Slashing		Heavy, Two-Handed

Weight: 6lb

Item type

Weapon, Melee

Rarity

Unique

Weight

6lbs

Base Price

Priceless

Raw materials & Components

Black iron, blackwood