"Be careful of that one." the arms master warned the princeling.
"Why?" his still too high voice asked as he ran his fingertips along the leather wrapped grip.
"Cursed!?!" the boy snatched his fingers away.
"Well..." he hedged "Not cursed, exactly. But it's vicious! It's been known to draw the strength of it's wielder to use against their opponent, whether the man holding the blade wills it or not."
"But shouldn't a man use his strength to defeat an opponent?"
"Use? Yes. Drain away, leaving him weakened as if he'd been sliced by his own blade? No... not that."
Blade of Woe
This magical blade begins combat with a +1 bonus to attack and damage. For every round it faces the same opponent it gains an additional +1, however that additional power is drawn from the bearer's own hit points, drawing 1 the second round, 2 the third, and so on. These lost hit points can only be healed magically. When a new opponent is attacked, the enchantment reverts to +1 and the process begins again.
Prop by Folkenstal, and based on the dagger from Skyrim.