User blog comment:OldManJenikins/Souls.../@comment-4674838-20120107001247

You need to be in a Hard Mode world where the Wall of Flesh has been defeated in order to obtain souls. Since you mentioned corruption monsters, it sounds to me like you're trying to get Soul of Night. The are dropped by any monster that dies in the Underground Corruption. In other words, you need to kill monsters underground, near lots of ebonstone, and in hard mode. Read more about Souls here.