The Trebithene Blade

Art source.

The Trebithene Blade is the Stradivarius of swords, forged nearly three hundred years ago by Ganar of Grösh, the greatest weaponsmith of the age. Though Ganar created over a dozen magical weapons at the height of his craft, the Trebithene Blade is by far the finest. Her metal is alloyed with ore stolen from the smith-god's forge. The runes etched into her blade promise death in the language of the underworld. Most puissantly, Ganar trapped an archangel in the pommel. During combat, the angel sings terrifying hymns (everybody can hear this). The rest of the time it pleads to be set free (only the wielder can hear this).

It is called Trebithene because the blade was commissioned by the Duke of Trebithene, who used the blade in his city's struggle for independence. During the post-war period known as The Lenient Reprisals, the blade was stolen by an assassin and removed from the city. From there, the blade pops up in history every dozen years or so, always in the hands of a great fighter.

Hooks And Problems

Most people have heard of the Trebithene Blade, and definitely every fighter has. It's still on the Trebithene flag! If you are seen or known to possess it, a fighter of your level will eventually appear to challenge you to a duel to the death for it. This reoccurs every 1d4 weeks in nature, or 1d4 days in a city or large town. If you ever conduct yourself dishonorably in such a duel and word gets out, future challengers will stop relying on honorable combat and attempt to ambush you while your party is busy fighting other monsters, or else use ransom and theft. It's not that the sword is cursed, it's just so famous that people can't help but take their shot.

Your players probably find the sword in the hoard of some big dragon-style creature, still clutched in the now-skeletal hands of the last guy to own it.

Without Ambriel around, the tzitzimimeh that are normally stuck in the 
mesosphere can come down to eat people whenever they want. (Source)

The Angel In The Pommel

The archangel in the pommel is named Ambriel. Her sacred task is to protect humans from malicious stellar entities. Like, if your campaign world has an evil star that whispers recipes for destroying the planet, it's Ambriel's job to stop that. She also tries to minimize the madness that can be caused by evil astrological influences. Things have gotten really ugly during the three centuries that she has been stuck in this stupid sword.

For example, the tzitzimimeh are a swarm of impossibly evil, vaguely astrological creatures that infest the regions between stars. They spend millennia swimming the void in search of inhabited worlds from which to feed. Since Ambriel has left the picture, more and more of them are showing up.

Another problem is the madness inflicted by unlucky stars. In the normal course of events, Ambriel will wander the world, invisibly curing or warding off zodiacal inflictions of this nature, or even killing people born under particularly evil signs. For most of history, she has kept things running smoothly, but she has been gone so long that there are entire cults of star-worshippers working all kinds of unseemly magic. For example, Ambriel senses that the yellowcake-priests of the Vapid Desert have almost built a working Time Rattleback with the help of The Star That Speaks.

Ambriel is acutely aware of the problems resulting from her absence. She'll try to explain the situation to the players and beg them to free her and destroy the sword. The blade's indestructibility will prove problematic even if a player is unusually willing to listen to a suicidal talking sword. Maybe the Frog-God's obliterating gullet could dissolve it. The caustic tears of the Vornish Pope would likely suffice to corrode it, but good luck getting that guy to cry. I'm sure your players will have their own ideas.

The smith-god's angels don't fuck around.

The Smith-God's Metal

Three centuries is nothing to Vhalkana. Never the most social or emotionally intelligent of gods, he can nurse a grudge for literally forever. He wants the sword back because it has his ore in it, because he's curious what the mortals did with it, and because he's angry that the sword is so tacky looking to his rarified, apollonian tastes. He is also secretly afraid that it might be superior to some of his work. The quicker it is removed from mortal hands, the better.

The master thief that snuck into his godsmithy and filched the ore is long dead, his soul well-hidden. Ganar Of Grösh died of respiratory disease at an advanced age, and Vhalkana has placed his soul in the hottest part of his forge, but that is insufficient to diminish the anger of a divine being. He has worked out an arrangement with the other gods so that he receives the soul of anybody who ever wields the sword, whose souls are then smelted down and used in Vhalkana's many projects.

Stuff the divine blacksmith might do:

• Send the sword-holder dreams in which a titanic figure places them on an anvil and demands that the sword be brought to their temple. The temple is somewhere really inconvenient, like it's called The Lightning Caldera, and in addition to being a mostly-dormant volcano it also is the place struck most often by lightning on the planet. When Vhalkana is in residence he uses the lightning to superheat his forge.
• Curse the sword-bearer to be unable to make anything. No item crafting, obviously, but also unable to make so much as a sandwich.
• Curse the player so that any nonmagical objects they possess break at the slightest provocation. 
• When he has time, Vhalkhana will sometimes make some angels and send them to retrieve the sword. The forge-angels have four faces. One of the heads chants blessings and heals. Another has a breath weapon that expectorates exquisitely formed, high-velocity swords and daggers. A third head curses foes with vertigo and palsies. The fourth head has a significant bonus to diplomacy checks but can't ever get the other heads to let it talk before attacking.