These are curses on weapons that are not about -2 to hit or damage, but more about limiting the fighter tactically. It may even be interesting to have weapons that have a bonus to hit and one of these curses and then let the player decide whether using the weapon is worth the trouble. This weapon is cursed so that you must always:
1. attack last, after all allies and foes. [Sloth]
2. attack only once per battle. [Sloth]
3. attack only until you wound a foe, then stop. [Sloth]
4. attack a foe only after it has been wounded by an ally. [Sloth][Envy]
5. attack only one foe until it is dead and keep beating the corpse until the battle has ended. [Wrath]
6. attack a foe, switch targets and attack that foe, switch targets etc, until the battle has ended. [Wrath]
7. attack only foes that are also being attacked by an ally. [Envy]
8. attack only foes that have attacked you. [Vainglory]
9. attack only foes that have attacked an ally. [Sloth]
10. attack only foes that have wounded you. [Vainglory]
11. attack only the last foe to have wounded you. [Vainglory][Wrath]
12. attack only foes that have wounded an ally. [Sloth][Wrath]
13. attack only when unarmored and shieldless. [Vainglory]
14. attack only while allies fight alongside, otherwise you surrender. [Sloth]
15. attack only alone, never with others. [Vainglory]
16. attack only unaware targets. [Sloth][Cowardice]
17. attack only after shouting your intended target. [Vainglory]
18. attack only the most threatening foe—the largest, most experienced—then stop when they are dealt with. [Vainglory]
19. attack only the weakest foe—the smallest, most frightened—then stop when they are dealt with. [Sloth][Cowardice]
20. attack only when begged by an ally. [Vainglory]
For some reason I started thinking of these curses in a framework of traditional Christian sins, like the knight that wielded the magic sword last was an anti-saint that was reeking of pride. Sorry if that's a distraction, there are other ways to frame curses as “bad” as I realized when I noticed that cowardice, which is often reviled, is not one of the seven deadly sins, nor selfishness. Also, I like the term vainglory here better than pride for boastful, showy fighting.