Just dump 'em in your setting. Careful: A magical weapon is both a curse and a boon. And a pretty great one of each.
1. The Original Weapon
What it looks like:
A rock, flint, a bit larger than a fist. One side is rounded. The other side has been chipped into a basic sharp edge. There is blood on it that never seems to wash off or dry out.
How it was made:
It was the first weapon crafted with the intent to murder another human being. Kain slew Abel with it. Romulus his brother Remus. It is murder incarnate.
What it wants:
It wants to murder. Not kill, it is not a weapon for combat. It is a tool for murdering. Those who trust you. Those who won't fight back.
What it does:
Any attack on a trusting target will always hit and will always kill. If you were to wield it in battle, you could only use it to hit your friends in the back of the head, not the enemy. For such purposes, it is absolutely deadly though. The only way to make sure this weapon cannot kill you is to never really trust anyone.
2. The Great Khan's Blood Blade
What it looks like:
A sabre of the type that the nomad warriors of the steppe wield as side weapons when riding into battle. The grip is made from human bone, wrapped in human leather. The blade is dark steel.
How it was made:
The Great Khan wanted to make a weapon from the pain of all who opposed him. It took the blood of nearly a quarter million people to extract enough iron for the weapon. In the end, it killed him.
What it wants:
Vengeance and pain are what it revels in. The anguish off all those who died in the process of making it will never rest and the blade wants to spread their misery.
What it does:
Hit a living thing and it will utterly desintegrate. Any small cut will suffice to turn an enemy into red mist. The weapon is also as sharp as a blade made from steel can get and remains indestructible to a degree that also prevents it from ever blunting.
3. The Chaos Flayer
What it looks like:
A Macuahuitl but grotesquely alive: There is an eye on the side near the tip, the whole thing is made from bone and the grip has living skin. Along the edges, human-looking teeth are gnashing.
How it was made:
The famous chaos wizard Tristram prayed to the Beings Beyond for his son to be the deadliest warrior to have ever lived. When the child was born, it looked normal but upon puberty, the boy developed a hunchback. In the end, a teratome or devoured twin grew out of his back, separating, killing the boy. This was the Chaos Flayer, a living weapon.
What it wants:
It wants to kill more people than anyone ever did. That's a lot (see the Great Khan's Blood Blade) but the weapon is alive and immortal and has all the time in the world. Also, as it needs human blood for nourishment, it needs to kill at least one person per day.
What it does:
It quadruples the strength of its wearer and will easily rip through anything that isn't harder than human teeth. Also, the effect of its cutting is identical to that of a chainsaw, ripping, rending, being generally very messy.
4. Natnarin's Boomerang
What it looks like:
A boomerang, carved from a material that is light as wood but harder than most ceramics while being slightly flexible. There is text etched into its surface besinging the Great Huntress of the North.
How it was made:
A craftsman who was in love with the legendary huntress got a hold of the tooth of a wyrm she had slain. For twenty years he toiled, grinding the material into the perfect throwing weapon. When he presented it to her, both of them old and worn by years, she granted him a kiss on the cheek as a token of gratitude. He commited suicide immedeatly afterwards as he didn't want to lose the feeling of her lips on his skin.
What it wants:
It wants to hunt, preferably rare monsters that are difficult to kill and are a threat not only to the hunter but to people in general.
What it does:
When thrown by a skilled hunter, this weapon will always return to them, even after hitting. It is effectively indestructable and the sharpened edge can slice through anything softer than wyrm enamel, which includes steel. It doesn't miss unless there is a good reason for it to want to miss.
5. Saint Helena's Hammer
What it looks like:
A full steel warhammer that culminates in a hand that holds a stake, all made from steel. It contains the actual bones of the right forearm and hand of Saint Helena, the patron saint of those who hunt witches, undead, demons and unholy monsters.
How it was made:
After Saint Helena died, the weapon was forged by the church as both a relinquiary and a weapon. While the rest of her body was interred in a cathedral in the Holy Capital, this weapon was made to continue her mission to cleanse the world from supernatural evil.
What it wants:
To cleanse the world from supernatural evil. In fact, the wielder won't be able to do anything else with it and it would probably punish them for attempting to do so.
What it does:
It starts to shiver whenever supernatural evil is around. It gives off a faint, silvery glow in the darkness - enough to use as a low-end light source. It will destroy any evil it hits, whether that be corporeal or incorporeal. If the metal stake is slammed through something and into a surface, it can be used to hold to question even incorporeal demons. You can perform an exorcism with it, by staking the victim through the hand, driving the demon from the body whilst trapping it.
6. Shiftblade
What it looks like:
It comes in a variety of shapes and forms but all in all it is always fully made out of silver- or mercury-looking metal. It has a total Mass of three pounds, which doesn't change. Other than that in between a rapier, a spear, a thin but strong metal shield, a flail or whatever you can think of, the weapon can be whatever the wearer wants it to be.
How it was made:
The soul of an innocent child that was murdered by his own mother was captured and bound into a vial of mercury by an evil alchaemist. This is how a shiftblade is created – after the first wielder forces their will unto that poor, tortured soul.
What it wants:
Actually it wants its suffering to end. If the wielder ever slips in their dominance of the blade, the Shiftblade will betray them, hoping for a release when their tormentor dies. It is, after all, an innocent soul held hostage by a violent individual.
What it does:
When someone picks up the Shiftblade, they must assert mental dominance over it in order to use it. Once that is done, it can be used in battle - taking any shape that the wielder wants in a flow that takes less time than the blink of an eye. Of course, blocking a blow with a shield that then becomes a spear to stab with and then a blade to deflect a counter attack is extremely effective - if you know what you're doing. Thus this weapon multiplies the base fighting ability of its wielder, making a skilled warrior profit far more than a clumsy beginner.
7. Twin Succubus Daggers
What it looks like:
Two curved daggers, the handles, made out of a light and smooth stone, carved into naked women. Surprisingly ergonomic though. The two weapons are connected by their pommels with a length of indestructible chain about one meter long so they must always be wielded together.
How it was made:
Someone sacrificed their soul to bind two succubi into an ordinary pair of weapons, which could have started out as something as mundane as butter knives. Then a blacksmith forged them into these weapons while the succubi were still bound, which must have taken some more sacrifices of human souls to accomplish.
What it wants:
Sex and murder, preferrably a combination of both. This is a weapon to bind and rape and kill someone with. Yet all parties involved will feel pleasure from whatever combination of sexual debauchery, sex crime and violence/combat is had with the weapon. Do involve the chain for some breath-play.
What it does:
It makes the wielder into a sexual combatant with lightning-quick reflexes and an appetite for both slaughter and sex. Effectively tripling the speed at which you can act. In anything like sex or combat it makes you a terrible monster to encounter. You will only remember things as a haze of red but at that point your soul is probably already on its way to hell.
8. Meteoric Khriss
What it looks like:
A snaking blade made from a dark steel. The grip is wooden and has a finely decorated pommel. It comes with a wooden sheath that has a protective metal bar by its opening and doubles as a smallish war hammer if you need a secondary weapon. The blade is about a foot long. It is of the hardest conceivable steel.
How it was made:
A prince of the island kingdoms long a go witnessed the fall of a meteor. As it was in an age before the advent of steel, the gift from the gods was forged into this very weapon with the express purpose of being able to combat not only human enemies but also supernatural beings.
What it wants:
It wants to help bring balance to the universe. Whenever evil is in the advantage, it will want to slice that to bits. But once the tide of a battle is turning, so will its loyalties.
What it does:
Cuts anything - even souls. If two people are bound by a link of souls it can slice through that if you can guesstimate where their connection runs through the aether. Cuts made by the curved blade will also heal very slowly.
9. Leech Dagger
What it looks like:
A roundish, hollow tube with a sharp point like a very large but a bit stubby hypodermic needle. It appears to be made from silvery copper. From the base of the blade, there are six curved metal pipes about the thickness of straws that wind around and end in sharpened points where the wielder will have their hand and wrist. The grip is spiralled with orichalcum, mythril and amber.
How it was made:
The Master Alchaemist's second apprentice, the one who turned evil, forged this weapon in order to give it to his lover, Shireigh the Assassin. He had hoped that the infused essences would keep her as beautiful and deadly as in her prime. In the end, it would be the blade that killed him and, much later, her as well.
What it wants:
Drink blood and infuse its wearer with it.
What it does:
Equipping it will immedeatly hurt you. Stab someone with it and it sucks out far more of their blood than they would normally lose and infuses it into you, whilst magically cleansing it and making it compatible with your body. It steals life force but that also contains parts of your victim's soul. Do it often and you will lose yourself in a sea of inner voices.
10. Rock Bian of Heavenly Justice
What it looks like:
A ridged, straight rod with a grip, all carved from a bone-coloured stone. It is ridged to look a bit like a human spine. Between the lowest ridge and the grip there is a ring of symbols that can be rotated around the shaft, although it was made from the same rock. Spinning it emits a very high pitched whine as long as it keeps turning, which can be up to half a minute.
How it was made:
It was carved out of bedrock from mount meru by a demon tasked to do so by a long forgotten emperor. Said emperor wanted a weapon that could smash but not cut as he had been prophesied to die by beheading with his own blade. The legend has it that it is one of the very rare intances where fate was actually averted: The emperor died of a stroke in his bath.
What it wants:
Smash the forces of chaos and disorder in the world. It does not care if you are good or bad, only if you obey a very hierarchical world view and know your place in it.
What it does:
While the lowest ring segment is spinning, hitting hard things with this will break and shatter them: The bone in your arm will break much easier than normally, it rips open armour and splits rocks. Swords connecting with it will splinter into pieces.
