In the original I.C.E. module Hillmen of the Trollshaws, on which this adventure is more-or-less loosely based, the room is filled with symbols of Utterdarkness, very potent magic from the Rolemaster rules. I adapted it like this:
"Magical darkness fills this large room, emanating from over a hundred arcane runes inscribed on the floor, walls and ceiling. (For the purposes of Dispel Magic, assume the runes were set by an 18th-level magic-user.) Mundane light sources are utterly useless here. Darkvision does not function. Magical illumination (Light spells, magic swords, etc.) will only work if the caster or wielder makes a successful saving throw vs. spells."
Fortunately, Yreorraent had a scroll with Light on it. (Rolled randomly at chargen.) Also fortunately, he made his save. Otherwise it would have been difficult and very dangerous for the party to proceed this way. I admit, the sadistic Referee in me was rather hoping they would be ambushed by the spiders in total darkness.