That's common sense, elementary internet knowledge and upper-intermediate history knowledge.
He was the sole factor that drove Führer to his insanity and detachment from reality(although you can't really blame him), and, as such, the true, albeit perhaps unintentional driving force behind many of those related historic events that began in first half of the 20th century, most of them being horrible events that most of the decent world is rather unconfortable and reluctant to remember. He's also (arguably)confirmed to having had the limited ability of omnipresence.
* It should be noted, however, that even today, it is still unconfirmed whether that is this mighty manipulative troll entity's real name. In fact, not even its final status is completely certain, although many believe it to be gone for good.
Because from the moment Fegelein is concerned, you cannot really be sure of anything anymore, amirite? *wink*
Günsche: Obergruppenführer, der Führer wünschet mit Sie sprechen.
Fegelein: Ich bin nicht in die Bunkeranlage.
Günsche: Sie sind in die Bunkeranlage. Sie sitzen direkt vor mir.
Fegelein: Wie bitte?
*momentary silence and very characteristic stares*
*Günsche walks out of the room*
*suddenly, thundering through the whole bunker* "FEGELEIN!!! *SLAM* FEGELEIN!! *SLAM* FEGELEIN! *SLAM*"