I was mostly going to go with a combination of Mayan/Aztec
legends and have The Power Key be the key to controlling the Earth's elements. It was used by the gods to destroy the Ancients, the original people of the Earth, and was the cause of the Great Flood mentioned in so many creation tales - including the Bible.
The Nazis in the new game are not just after the devastating power of this Key, but they also wish to resurrect the Ancients - who they believe to be the original Aryan Master Race they themselves originated from.