Corrupted Spiders are giant arachnids which grow in the depths of the Deep Roads, and feed on numerous species of large bats. When the Deep Roads were lost to the darkspawn, they started feeding on genlocks, and their numbers began to grow exponentially. Some moved up to make their lairs in surface forests, but most remain underground, close to their blight-tainted meals which make them larger and fiercer than they have ever been.
--From Codex entry: Giant Spider