Dungeon Master: Bloodlust Weapon

A weapon craving the blood of a target is quite a popular archetype. However, to make such a thing a reality in table top role-playing games, we need to use fluff and mechanics. What I will do today is put forward several different ways to make such a weapon a reality. I also want to mention that I would usually avoid giving this kind of weapon to a player in most forms. Taking away options from the players usually makes things less fun, not more. I will also focus on the kind of weapon that will want to focus on a single target after it tastes it's blood and not the kind of weapon that likes to target one kind of enemy (Dungeons & Dragons 5^th edition has enough of that).

When the weapon tastes the blood of a creature, it will have an overwhelming urge to finish the job. Until that creature lies dead or the craving subsides, the wielding of the weapon will feel the push towards that one target.

The Carrot

One way to make such a weapon is by giving an incentive for the character to attack the target. By giving a static bonus to hit and attack or a dice roll for extra damage against the creature the weapon has targeted, you don't take away the characters options but give an incentive to go after a certain target. This is probably the only type that I would consider giving a player as an actual beneficial magic item (the rest are more like cursed items).

Variant 1: After a character hits a creature using this weapon (addition restriction could be that it needs to bleed), any subsequent attack against this target gives a bonus +2 to hit and damage. The target must die or be out of visual range before a new target can be chosen.

Variant 2: After a character hits a creature using this weapon (addition restriction could be that it needs to bleed), any subsequent attack against this target gives a bonus 1d6 to damage. The target must die or be out of visual range before a new target can be chosen.

The Stick

Of course, we can try to punish a character for not attacking the correct target instead. There are multiple ways to do so including a penalty to hit, damage taken for attacking the wrong target or even not giving a choice in the matter. If given to players, they will act more like cursed items.

Variant 1: After a character hits a creature using this weapon (addition restriction could be that it needs to bleed), the creature that was hit becomes the target. An attack against anyone but the target will have disadvantage. Killing the target or being out of visual range makes the creature that was hit no longer the target.

Variant 2: After a character hits a creature using this weapon (addition restriction could be that it needs to bleed), the creature that was hit becomes the target. An attack against anyone but the target will result in 1d6 damage to the character wielding the weapon. The target must die or be out of visual range before a new target can be chosen.

Variant 3: After a character hits a creature using this weapon (addition restriction could be that it needs to bleed), the creature that was hit becomes the target. The character wielding the weapon is overcome with an intense blood lust and cannot willingly move farther away from the target. They can also not attack anyone but their target.

Note: I'd be hesitant of even using this as a cursed item but it works well in the hands of an NPC.