The Abyss (1989)
- He who fights 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.
Another such variation being:
- Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you.