Drow Long Sword

Drow Longsword is an enchanted longsword +3, which dissolves to Adamantine Dust when its taken outside the Underdark, like all drow equipment. It inflicts 1d8 +3 points of slashing damage, has -3 THACO bonus, a speed factor of 2 and it weighs 4 lbs. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition.

It's regulary dropped by drow warriors who can be found all over the Underdark.

In-game description
"Drow Longsword +3 These swords are usually referred to as doubled-edged swords, war swords, or military swords. In many cases, the long sword has a single-edged blade. Most long swords have a double-edged blade and a sharp point at the tip. Despite the tip, the long sword is designed for slashing, not thrusting. This item, like most drow equipment, is created using Adamantine, an alloy of adamantite that quickly turns to dust if removed from the Underdark."