Thought I'd throw one at you as a freebie
The Sword of Vengeance
These weapons tend to show up during particularly bitter wars and eras of strife. Whether planted there by deities of war and hatred, or spontaneously reacting to an overflow of emotions, they become sought after devices of war.
While most are long swords or bastard swords, they may take the shape of any sword. In combat, they strike with a +1 bonus to hit and damage.
However, whenever the wielder is hit during melee combat, the wielder immediately receives a free attack in retaliation against the enemy that struck him.
If a natural 20 is rolled on any attack roll (regular or in retaliation) with a Sword of Vengeance, the blade will emit a piercing shrieking sound. The wielder immediately loses 1 hit point per hit die, and every enemy within 5 feet takes the same damage (typically melee range)
For example, a fourth level Fighter would lose 4 HP, as would the targets, in addition to any regular damage inflicted.
Let me know in comments what you think.