SS-Obergruppenführer Reinhard Heydrich was intimidating at six feet three inches tall, which terrified everyone. Only the Führer and Himmler were in more command than the Nazi leader.
It was rumored that Heydrich would eventually succeed the Führer as the leader. Heydrich was promoted to lead the Gestapo because of his success in creating the Nazi party intelligence service (the SD), an unforgiving apparatus of spies and terror (the Nazi secret police). He was one of the most effective National Socialists, one of the staunchest champions of the concept of the German Reich, and one of the most ferocious opponents of the Reich. The ominous reputation of the Nazi secret police as being capricious and callous was deserved. Heydrich’s death led to mourning across the Nazi empire.
Source: https://www.wordonfire.org/articles/fellows/dark-lessons-learned-from-the-nazi-butcher-of-prague/