Probably the meanest cursed sword I can think of

It looks like a normal sword, of course. Doesn't detect as magic, immune to remove curse, dispel evil, etc. etc. 

When a PC picks it up, tell the player to pick another player at the table. Don't give them any context—just tell them to choose. They cannot choose themselves. 

An unsightly black mark forms on the forehead of the chosen player's character. The mark gives the character -1 to attack and damage rolls, and the bearer of the sword gets +1 to both. Additionally, 10% of all xp the marked character were to earn goes to the bearer. 

If the character who bears the sword were to die, the marked character dies instead. 

When this happens, the player whose character holds the sword chooses a player again. Both that player's character and the previously chosen player's new character receives a mark and penalty, and the bonus to the bearer increases to +2. The same player can be chosen again, meaning their character gains an additional mark and a cumulative penalty. If the bearer of the sword were to die and more than one character is marked, the player chooses which one dies instead.

This process repeats until every character at the table is marked save for the bearer, or a total of six marks have been distributed. The next time a situation were to occur where a PC would be chosen to gain a mark, tell the player to pick another player at the table as normal. When a new player is chosen, all pre-existing marks disappear. The sword immediately transfers to the chosen player's character, and a mark forms on the forehead of the previous bearer. The process begins anew, with the previous bearer being the first marked character.