Unwholesome Blades come in many shapes and sizes, ranging from that of dagger size, up to (rarely) that of a two-handed sword. These foul weapons will inflict the usual amount of damage to enemies for their weapon type, but also have two other characteristics in common: they are +1 to Hit and Damage, and they will inflict a Cause Light Wounds spell (1d8 +1 HP of damage) upon their wielder, each and every time they are used to inflict damage upon any enemy. Undead monsters using one of these blades are actually healed by it for the same amount of points that a Cure Light Wounds would give them.
Getting rid of an Unwholesome Blade can be difficult. Throwing it away is of no use; if the wielder does so, the blade will always magically come back, up to and including replacing whatever weapon the wielder chose to use, right in the middle of combat, no matter how far away it was left behind ("I attack the monster with my short bow." "Actually, you no longer seem to have a short bow, but there is a familiar-looking sword in your hand now.")
Destroying the blade is one possibility, though this can be risky: if broken or ruined in some fashion, it will inflict a parting 1d8 +1 HP of damage from Cause Light Wounds upon its owner/wielder, even if separated from them (the only time it can do this from a distance). As a magical item, it gets a +1 bonus on all Saving Throws against any such attempt. A successful Remove Curse spell will allow the wielder to be rid of it without any parting damage, though the sword will still be cursed.
If destroyed by someone who is not its wielder, it will not inflict magical damage, though it still gets its +1 bonus to Saving Throws.