It's called a Wechselbalg :)
In european mythology a Wechselbalg was a baby, wich was planted by the devil in exchange for the real one shortly after its birth (Balg beeing a very old word for a baby or Säugling (to quote Monroe XDD) and Wechsel means exchange, replacement, swap). Even though it sounds like it, the term has primarily nothing to do with changing forms. It just means "replacement (devil) baby".
The book refers more to the Doppelgänger-legend, in wich you die, when you see your double (in this case, because the Wechselbalg takes everything away to become you and destroing the original in the process...)
Finished Chapter 4 this morning on my way to work. So far its a good read. And the author's done his homework. The german phrase used in Chap4 is actually correct XDD
Really nicely done with Verfallserscheinung. I like the idea and the very accurate meaning behind it. Its a really fitting term for this case, though I would describe it as a state of decaying or 'cell fatigue' rather than dementia...
btw, since when does Nick have an enhanced sense of smell? The book isn't canon, right? So I guess the author just added it to Nicks special Grimm-Talents, but it makes sense in a way. Enhanced hearing, enhanced physical abilities, enhanced mental abilities, so why not enhanced sense of smell...