In OTL, Reinhard Heydrich was a high-ranking Nazi who happened to be not only unscrupulous, but dangerously competent. While this is based on reality (he didn't become Himmler's right-hand man for nothing, together with Rommel and Yamamoto he was one of the few nazis the Allies actively tried to kill, and Göring coined the saying “Himmlers Hirn heißt Heydrich” - “Himmler's brain is named Heydrich”), in “Nazis win” AH (especially books), it's become a cliche that a Nazi victory is outright impossible if he doesn't survive. Often, he even becomes the next “führer”.