A part that I left out of his background is that since the forest is shared with Germany, he terrorized German hikers and campers in the forest due to him blaming Germans for his injury and
I guess it depends on his base of operations. If he operates in Germany it makes (a little) sense – otherwise not really, since not every polish speaks german like not every american speaks spanish. That part of the forest is called “Ueckermünder Heide” by the way … 
that is what the Germans called him.
The problem with “Teufling” is that it sounds exactly like the real existing word “Täufling” which means “baptizand” – so prettty much the exact oppsite of your intention wherefor no real German would call him that. I guess they would simply call him “ein Teufel” (the word can also be used in a more general sense for anything diabolic appearing) or maybe “Dämon” (demon).
But of course you can name your characters however you want…
Teufel is German for devil and is the basis for Tiefling in DnD so I chose Teufling which is a made up word. I speak no German but it fit the concept.
I live in Germany for 31 years now and grew up quadrilingual (luxembourgian, german, french and english). I still wonder sometimes how “hard” and thus somewhat inflexible the german language is and find it hard to make up convincing sounding new words.
I just got tired of writing the background story and it always sounds much better in my head than on paper.
I really didn’t want to patronize you and I apologize if I seem like a smartass.
But even if I’m not a native speaker I always had a slight problem with the way how ignorant some Americans treats the german language – worst examples are Marvels “Nightcrawler” (a supposed bavarian – guess where I live) and when Kiefer Sutherland aka Jack Bauer masqueraded as a german arms dealer in the last season of “24”.
That last one really was a nightmare since there was in fact a german actor on the set with Jürgen Prochnow (he played a russian mobster of course) who could have corrected the german dialog with ease. But why bother? I know it’s not the end of the world, but it’s a little annoying if it happens over and over again.
