The Phase Spider is one of three different high-level Spiders encountered in the original Baldur's Gate, the other two being the Wraith 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 bright-green body and dark-green legs - pointedly different from the much more common Giant Spider.
Like most spiders, they can Poison their enemies and are immune to Web. It's essentially a more powerful Giant Spider, with the unique ability to quickly 'blink' short distances, likely much closer to Gorion's Ward than they'd otherwise prefer. Unlike most spiders which can normally pulled by a specific party member and/or drawn into a trap, the Phase Spider will often teleport behind or within the parties formation, likely striking (and poisoning) a few weaker party members before the player is able to dispatch it. 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.