Can you provide me with a famous "stare into the abyss" quote and explain its meaning?

The famous quote "He who fights with monsters should look to it that he himself does not become a monster. And if you gaze long into an abyss, the abyss also gazes into you" by Friedrich Nietzsche warns that when confronting evil or darkness, one must be cautious not to be consumed by it. It suggests that prolonged exposure to negativity can have a corrupting influence on a person's own character.