A long sword burnished black, glinting with a hint of silver. It does 1d8 damage and grants no bonus to hit. But, any creature hit by the sword must make a save vs. poison or be instantly slain. It's soul bound into the sword. So there is no way to resurrect a person while the sword is still whole. The lost dweomer might get around this limitation.
For any wielder, the sword will 99 times. The 100th death is the death of the wielder. After slaying the 99th victim, each time the sword hits another person, the wielder must save vs. poison or die. His soul is then absorbed into the sword.
Of course clever players would probably only use the sword, when it really counts. But alas, escaping the curse is not that easy. Each time the wielder is reaching for another weapon, he must succeed at a save vs. magical device at -5. If he fails, the weapon he grabs, is the Sword of 100 Deaths. Of course, this effect also means that it is extremly difficult to separate the wielder from his weapon.