I just noticed this website, and offer a few bits of info that might be of interest for game purposes:
1. In the original 1581 account, a couple of details were left out from the Wikipedia retelling. The first is that the murdered accomplices of Genipperteinga were thrown into a mine shaft within his cave complex.
2. It is also stated how dangerous Genipperteinga was, parties consisting of 3, 4 or even 5 persons might not be safe from him. He had lots of armor, swords and musketry at his disposal, so I assume his modus operandi was to attack his victims as a fully armored knight, or something like that (the text does not substantiate that modus operandi, but is a conclusion I reach knowing he could overpower and kill 5 individuals single-handedly, and his ready access to arms)
3. The area were he is said to have lived IS riddled with caves, I came across a photograph from that area here:
At a website called panoramio ("dot" com) slash "photo" slash 60478563
Cheers