The origins of these accursed swords are shrouded in mystery. Both long swords and short swords appear in historical accounts, all of them very cleverly made to give the appearance of being powerful elven weapons.
Clerics or magic-users who pick up a Vancian Sword suffer 1d6 of electricity damage per round until they let go. These characters will also get the distinct impression that a sweet elven voice whispers to them about sharing "their gift" with a powerful fighter or thief.
Fighters or thieves who pick up a Vancian Sword must save versus spells or be stricken by the curse. Those who make the save suffer 2d6 of ice damage per round until they let go. These characters will also hear an angry elven voice chide them about not being worthy of "the gift" the sword has to offer. Those who fail their saving throw will hear a sadistic voice, decidedly human in character, laugh and cackle diabolically. Despite feeling ashamed and insecure when they hear the laugh, these characters will be unable to let go of the sword and will prefer it over all other weapons until a Remove Curse spell is cast on them.
Those using a Vancian Sword will fight rather strangely. Find the highest spell-level that a magic-user of the wielder's level can cast. The sword will strike unerringly that many times each day, no to-hit roll necessary. However, the target gets a saving throw versus magic for half damage. A Vancian Sword can hit creatures only harmed by magic weapons and does 1d6 regular damage as well as 1d6 acid damage on each hit.
After the sword's daily allowance of attacks has been used up, the wielder has to save versus spells for each additional attack. Success indicates that a normal to-hit roll can be made for 1d6 regular damage. Failure indicates that the wielder's morale fails: they will leave combat and try to hide behind fighters and clerics, begging for healing and support, quivering for their lives. The diabolic cackling continues in their heads until they can somehow leave combat and rest for at least one turn to calm down. Until the wielder gets 8 hours of sleep and another hour in the morning to "appreciate" their sword, they will have to keep making saves against spells for each attack with the same consequences for each failure.
(Optionally, while a Vancian Sword is being used, the characters around the wielder will hear a faint murmur of people arguing about magic systems.)