I basically base most of this stuff on D&D since that's where all the modern fantasy RPGs come from (which in turn descends from Tolkien of course). 

I rather assumed the Dredge was a typographical error since I couldn't find any reference to such a thing online and figured it was a Dretch. Dretches are demons from the Nine Hells and they "hate" paladins and all things "Good". Unfortunately, as demons and devils are not from our world they aren't vulnerable to our weapons. This is where clerics, priests and paladins come in handy. They have the ability to bless items or people and make them "Good" and demons and devils have a vulnerability to things that are "Good."

Needless to say, I'm quite confused now.