Giant spiders tend to appear in old ruins and other places where the 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 the Fade, tainting such creatures as these spiders to transform into larger and more potent creatures than 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.
Corrupted spiders are giant arachnids that originally grew in the depths of the Deep Roads, feeding on numerous species of large bats. When the Deep Roads were lost to the darkspawn, they began to feed on the numerous genlocks and their numbers grew exponentially... as did their size. The darkspawn taint has become a permanent part of their system, passed on to their progeny. This has had the effect of increasing their size abnormally, as well as their aggressiveness. Some corrupted spiders have made their lairs in surface forests, but most remain underground, close to their Blight-tainted meals.