These appear to be a bundle of twelve +2 arrows, which, if tested, will radiate magic, and if specifically tested for curses, will reveal their true nature (in practice, most groups don't seem to look for curses in arrows; that will likely change after this). Until used in a crisis, these arrows will appear to be entirely beneficial to their user, with magical bonuses to both hit and damage, along with the ability to strike creatures whom only magic will harm.
Upon being fired in actual combat, the arrows will curve around in mid-air and attempt to strike one of the archer's group, or even the archer themself! The GM rolls randomly to see which party member has been targeted. In this case, the arrow will strike as if the archer had intentionally been trying to hit this person, utilizing all of the archer's applied bonuses to Hit (if any).
Optional: Treacherous spears, sling bullets, or even a Wand of Fireballs are all possible, too!