What does this sentence mean? How can I say it differently?

The wording of your sentence is a little archaic, but it is certainly grammatically correct.

What might be making you stumble is "told of a prophecy," and possibly the word there.

Imagine the elders of a primitive tribe in the highlands of Scotland sitting around a campfire. Imagine one of the elders saying,

"My great, great, great, great, great grandfather told of a prophecy that has not yet been fulfilled. It concerned the tragic death of the seventh son of a future king of Scotland who would be slain by his own father, the king!"

In other words, the elder is looking back into his family tree to tell his peers about a prophecy that has been told again and again for generations but has not come to fruition, yet.

As for there, a sentence preceding your sentence could provide a referent for the word there. The sentence could be worded as follows:

The setting of the tale is Babylon, early in the first millennium, BCE (i.e., Before the Common Era). There the ancients told of a prophecy.