Seriously, this was interesting.
Ich spreche nicht sehr gut German, ja, aber some words are very close to the Swedish-svenskö-mumbo-jumbo language like; Abgrund --> Avgrund etc.
The word monster is the same in German/English/Swedish, but Nietzsche uses the word Ungeheuer, which afaik is more like behemoth/beast/cryptid... (
or 'the second beast' = false prophet = impostor?)
Here's my try:
"Wer mit Ungeheuern kämpft, mag zusehen, dass er nicht dabei zum Ungeheuer wird. Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein."
"He who fights with beasts should make sure, that in the process, he becomes not a beast. And when you gaze long into an abyss, the abyss gazes also into thee."
Devils Epilog:
Thou who fight with German geniuses, make sure before wasting too much time, that the man whose work you've gazed too long into, is not gazing into thee with a megalomaniac nutcase stare.
Bertrand Russell was a cool dude.