This enchanted shortsword has strange runes engraved all over it. The blade never needs to be sharpened as nothing seems to dull it. The blade will change to become almost anything that is more powerful than the weapon or opponent attacking the bearer.
On it's own as a shortsword, it is a +4 weapon to hit and to damage. When it changes to something else, the duration will last for however long it senses a threat to the bearer. Each blade has a natural enemy that it was specifically made to oppose. For example, one blade may be opposed to demons while another was made to oppose elves. Yet another might be made to oppose giants or dwarves or humans or just about any living/undead creature. While one of those it was made to specifically oppose is in the vicinity of the bearer, the blade will retain it's changed form and attack until all it opposes are destroyed, even if they might happen to be a PC's party member.
Each blade has a special control word that the bearer can say to stop it and return to it's original form, reappearing in the bearer's sheath.
This blade is especially prized by assassins and fighters.
The bearer will know that it is an enchanted blade if discovering it accidentally but unless they can read the runes inscribed on the weapon, they will not be able to learn the control word or know that it can change. A sage who can read runes of the Sartan must be consulted to gain that information. the name "Blade of the Sartan" is inscribed on the sheath in each of the humanoid languages.
The Sartan are a race of extremely powerful magic users who are now extinct. There are no Sartan left, only books that they wrote which sages may study to know of them.