A simple lesson, in a nutshell:
You might be very powerful, but in the end you’re not getting away with anything; mind your actions very carefully. You have certain powers in the world to do things, but power always comes with both responsibility and danger; this is unavoidable.
That’s the punch line. Here’s the story:

Damocles was pandering to Dionysius, his king, and said the king was fortunate, surrounded by magnificence. In response, the king offered to switch places for one day so that Damocles could taste that very fortune firsthand. Damocles quickly agreed. He sat down in the king’s throne surrounded by every luxury, but Dionysius, who had made many enemies, arranged that a huge sword hung above the throne, held at the pommel only by a single horse hair to evoke the sense of what it is like to be king: having much fortune, but always having to watch in fear and anxiety against dangers that might overtake him. Damocles finally begged the king that he be allowed to depart because he no longer wanted to be so fortunate, realizing that with great fortune and power comes also great danger.

This story may remind you of another classic masonic symbol: The “Sword Pointing to a Naked Heart” demonstrates that Justice will sooner or later overtake us, and that although our thoughts, words and actions may be hidden from the eyes of man, they are not hidden to the All-Seeing Eye. Rather than the danger of enemies being omnipresent, it is the perception of the All-Seeing Eye. The message of both swords boils down to this:
Don’t fool yourself into thinking that you are avoiding accountability. At best, you can evade it for a period of time.