They kill their enemies with their petrifying gaze, making Potions of Mirrored Eyes a necessity when fighting them, as a single basilisk can easily wipe out even a mid- to high-level party very quickly. Scrolls of Protection From Petrification can also be purchased and memorized by mages or bards. If the worst does happen, scrolls of Stone to Flesh can be bought to restore petrified party members provided the party member was not shattered.
The undead are notably resistant to basilisk's gaze, so animating some skeletons to absorb the gaze attacks is also a good idea. Korax is a friendly Ghoul in Area East of Temple that is intended to be used for this purpose.