The Equalizer

The Equalizer, also known as the Sword of Neutrality, is a longsword +3 in terms of enchantment. It has +3 bonus to thaco and can inflict 1d8 points of damage.This blade does extra damage and has a higher chance of hitting depending on the alignment of the creature. Alignments more extreme will suffers more damage and are more likely to be hit. Thaco and damage to: Also, wielding this longsword grants the wielder immunity from charm and confusion. This weapon can, however, be wielded by characters of all alignment with a strenght of minimal 6, but not by clerics, druids and mages. The Equalizer has a weight of 3 lbs and a speed factor of 3. This item appears in Baldur's Gate II: Shadows of Amn. It's a very powerful long sword, perhaps the most powerful.
 * True neutral: +0 to thaco and +0 to damage
 * Chaotic&lawful neutral: +1 to hit and +2 to damage.
 * Neutral good&evil: +2 to hit and +4 to damage
 * Other four alignments: +3 to hit and +6 damage.

It can be obtained if all three pieces of this weapon are brought to Cromwell in Athkatla Dock District. He will unite all pieces for 7500 gold. The three pieces are: This means the weapon can be obtained as early as in chapter 6 of the game.
 * Blade: can be found in the Mind Flayer's Dungeon in the Underdark.
 * Hilt: can be found in the Beholder's Lair in in the Underdark.
 * Pommel: can be found in Irenicus' Dungeon.

In-game description
"Such is the age of this sword that its true origin has been lost to time, but markings hint that it was possibly forged in the service of Helm. Also called the "Sword of Neutrality", it seems designed to seek and terminate extremes, to shift the universe closer to harmonious equilibrium. The further the behavior of a target from true balance, the more potent the damage they suffer."