prunelle:

“When you stare into the abyss the abyss stares back at you.”

— Friedrich Nietzsche
German philosopher (1844 - 1900)

However, I always preferred the alternate (and extended) translation. I think it was Walter Kauffman…?

“He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”