The Wraith Spider is one of three different high-level Spiders encountered in the original Baldur's Gate, the other two being the Phase Spider and Sword 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 yellow body with dark-purple legs.
Like most spiders, they are immune to Web, yet lack the gradual Poison that most other spiders are known for. Instead, despite it having relatively low health, it's immune to normal weapons - making it more than a challenge for most low-level parties. Furthermore, if this spider lands a successful hit, and its target fails a Save vs Spell, the target's Strength ability will be drained temporarily by 5 points. If the Wraith Spider lands multiple hits and lowers a single target's strength ability below 1, the target will instantly die, thus killing such a spider is generally a high priority - just remember to use magic.