The Black Legion is a lot like The Black Company from the Glen Cook’s series “The Black Company” and i believe it was kinda of based on that. Just like the “Gollu’s ring” from the “Lord of the Rings” 
(The Black Company is a series of dark fantasy books written by American author Glen Cook. The series combines elements of epic fantasy and dark fantasy as it follows an elite mercenary unit, The Black Company, through roughly forty years of its approximately four-hundred-year history.<Wiki )
I do recommend The Black Company books 
