What would be really mean would be for the characters to contract the Spider Sickness. I've left out the save DCs as this would be a really cruel thing to do to a PC without some kind of way back.
Spider Sickness is believed to be spread by bites from spiders. ((it is not. it is in fact spread through casual contact but the symptoms do not show up for 1d4 weeks)) The magical disease shows quicker in those with Elven blood but is not limited to Elves or Half-Elves. Humanoids that are not Medium sized are immune somehow.
Stage One: The first day the disease manifests the victim develops a sensitivity to sunlight, loses appitite, and gains a slight fever. Bathing in pure water while under the noonday sun will cure a victim at this stage, assumeing they know to do it. At this point the sickness can be misdiagnosed as Rotwood Fever. Cases of Spider Sickness are rare because the Elves are very thorough in cleaning them up. They quarentine entire forests if need be. Only sages or healers with experience would be able to determine the fevers true cause.
Stage Two: For those with Elvish blood this stage is reached within twelve hours. Other humanoids reach it after twenty four. The victim experiences terribly painful cramps. Non-elves rapidly lose weight until they reach the average weight for an elf that hieght. They do not lose weight if they are already at or below it. Non-Elvish victims also develop pointed ears at this stage. Stage two can be cured but it requires the victim to be forced to drink holy water that has be annointed by clerics of Corellon Larethian. Victims will not drink it willingly as it burns them.
Stage Three: This is reached three hours after Stage Two for Elves, six hours for others. At this point the victims skins turns black as midnight. All color leaves their hair and the whites of the eyes turn blood red. The victims mind is assaulted by visions of spiders and murder. They can no longer stand the light and feel an overwhelming urge to escape underground. The Elves usually put down Stage Three victims as they have been declared Anathema. Nothing but a Wish or Miracle can help them now. A few cases that go unreported do manage to escape sometimes. The Elves do not know what becomes of them nor do they greatly care to find out.