Kept in a dagger sheath, the Phantom Blade is the stump of a broken sword. Only a maimed swordsperson can wake the memory of its former wholeness. In the hand of such a fighter, this unwieldy knife becomes a +1 longsword for all purposes. Its ghostly steel only appears for a fraction of a second to slash or pierce.
Once per fight, the wielder can get advantage to initiative, attack, or parry (ie confer disadvantage to the attacker.)