The Sword Spider is one of three different high-level Spiders encountered in the original Baldur's Gate, the other two being the Wraith Spider and Phase Spider. They are one of the more interesting enemies to be found, as they exhibit some fairly deadly and unique abilities. They are fairly rare, usually only found within the depths of various forests, and are distinguishable from their brethren by way of a blue body with black legs.
Like most spiders, they are immune to Web, yet lack the gradual Poison that most other spiders are known for. Instead, this spider is naturally Hasted, able to close on its target in about a second and attack with incredible speed. The easiest course of action for the player is to distract the spider using a companion with a high AC vs piercing attacks and focus fire with the rest of the party. Most specimens encountered in-game are pre-generated, meaning that there likely won't be a consideration made for the strength of the player, making for a very difficult fight regarding unprepared low-level parties.
Player controlled Sword Spiders(summoned with the Spider Spawn spell, if the caster level is 12 or higher) can be extremely effective against enemy Mages; with four attacks per round they quickly chew through Mirror Images and Stoneskins, and then just keep on interrupting enemy spellcasting with every hit.