The cursed Helm of Opposite Alignment is a truly fearsome item. As its name suggests, it has the power to alter the fundamental ethical framework, nay, the very identity of anyone unlucky enough to wear it. It can ruin homes, friendships, even entire cities or nations. Should the legions of evil ever force one onto the head of a benevolent solar, the multiverse, as we know it, will surely meet an abrupt and painful end.
The alignment change happens on both the ethical scale and the moral scale: Evil-aligned creatures will become Good on equip while Lawful creatures will become Chaotic and vice versa, but creatures who are Neutral on one scale will remain Neutral on that scale, as such, True Neutral characters will experience no change in alignment
Although it seems like a useless item at a glance, it can be used to allow the wearer to equip alignment-restricted gear that would normally be out of their grasp such as Twinkle +3 of Drizzt's
The helmet allows companions to remain in party with reputation not to their liking, eg. Kagain or Shar-Teel in Heroic 19&20, Coran or Kivan in Despised 2&1; it also prevents (or causes) potential conflicts between companions, eg. Ajantis doesn't like Evil guys
It affects Turn Undead ability as well, Viconia with this helmet will explode skeletons instead