Giant spiders tend to appear in old ruins and other places where the Veil has become thin because of magical disturbances or a great number of deaths. In such places, spirits and demons pass into the world of the living and attempt to take control over living beings, spiders among them. Not all scholars accept this explanation for the presence of these beasts, however. Some claim that the thinning Veil allows magic to “leak” from the Fade, tainting such creatures as these spiders to transform into larger and more potent creatures then they ever would become naturally. While such spiders are known to possess powerful poisons and the ability to fling their webs at opponents in combat, studies of them have been few and the full range of their abilities are unknown.
—From Codex entry: Giant Spider