'The Key'...
Even though Leon didn't know what kind of 'key' it was, as the master of the Death Angels, he was well-acquainted with his 'birthplace', the place where he fought, which was the Knight Star.
It was the core device of the Knight Star, something he discovered after unifying the entire Knight Star.
Why were the forests on the Knight Star twisted? Why did they breed huge beasts far beyond what normal ecosystems could produce? Why was the awakening rate of Psychics particularly high? Why did mysterious disappearances happen frequently?
These answers remained unknown during the long human colonization of the Knight Star, but after he unified the planet, he certainly had to find out.
Especially after the planet returned to the Empire's rule and introduced Imperial technology, he further investigated it.
