Spider's Bane

Spider's Bane is a two-handed sword made to fight against spider more effective than normal weapons, because it bestows the wielder the free action status, making the wielder immune to magic that affect movement. It's obtained during the subquest: Finding Shelak in the Cloakwood Forest.

In-game description
"Spider's Bane was originally forged by the dwarves of the Orothair tribe in the Cloakwood Forest. They created the blade to help foster good will between them and the Grand Dukes of Baldur's Gate. Wielded by the Grand Dukes for about a century, the sword was eventually lost, ironically in a fight against ettercaps and spiders. Who wields the balde now is unknown, but its discovery would aid greatly in fighting the current spider infestation that plagues the Cloakwood."