Albruin

Albruin +1 is a bastard sword that grants immunity to poison and the ability to use Detect Invisibility once per day.

It first appeared in Baldur's Gate II: Shadows of Amn. Later it was added also in Baldur's Gate: Enhanced Edition, where it can be looted from Simmeon (who is part of Dorn Il-Khan's quest) and also counts as a silver weapon, making it useful against werewolves.

Baldur's Gate: Enhanced Edition

 * Wyrm's Crossing - Loot from Simmeon (part of Dorn's storyline)

Baldur's Gate II: Shadows of Amn

 * Underdark - Reward from wizard Raevilin Strathi if you free him from Imprisonment in the sphere and cure him from madness with a Heal spell after he attacks you

In-game description
"Basalin was a large man, rumored, far from his hearing, to be a werebear. He long wielded the sword Albruin in the protection of his village, slaying giant spiders that plagued the area. The sword protected him from their poison and also allowed him to see invisible Red Wizards who controlled some of the more fantastic variants. The reason for their enmity is unknown."