Soul Reaver

Soul Reaver is an enchanted two-handed sword +4 that can only be wielded by neutral and evil aligned warriors and bards. It can deal 1d10 +4 points of damage, has a -4 THACO bonus and a speed factor of 6. The weapon has the ability to give a cumulative THACO penalty of 2 each hit to enemies, which lasts 20 rounds. The weapon weighs 10 lbs. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition. It's gained as drop from one of the five demonknights near the shrine of Demogorgon in the Underdark.

In-game description
"Two Handed Sword +4: Soul Reaver A social faux pas placed upstart Andor Licon in a duel with Baron Eeirk, heir to a warrior king. Though they seemed of even skill, Andor grew oddly weaker with each hit of the Baron's family blade. Panicking, he bade his bodyguards kill Eeirk. Despite his claims of morality, some noted that the young noble had positioned his men before the duel was even underway, and though he professed disdain for its dark magic, Andor made a tidy fortune off the sale of the evil blade."