Your approach has stirred up the pit's occupants, and they suddenly scurry out to protest the invasion of their privacy. The delvers must defend themselves from the attack of four large spiders with a monster rating of 12 each (2D6+6). Any character not wearing armor must make a L1SR on Luck to see if he was bitten. The bite is poison and does 3 points of damage this turn, and three next turn.
Any character who actually takes hits from a spider attack is bitten, and takes double damage from poison. Fight the battle to the finish. If the spiders win then you are DEAD! If you kill all the spiders, you might want to investigate the pit.