Sybill Trelawney was a descendant of the renowned Seer Cassandra Trelawney.
Legend had it that Cassandra Trelawney had been cursed by Apollo: her prophecies were accurate, yet no one believed them.
And in the world of Harry Potter, this eccentric, rambling professor, Sybill, was something of a "major player" without actually being one.
Why did Shawn take Sybill's prophecies so seriously?
That was without even bringing up a freakishly gifted figure like Grindelwald.
After all, strictly speaking, the plot of Harry Potter was essentially the story of Sybill's prophecy.
The prophecy went roughly like this.
"The one with the power to vanquish the Dark Lord approaches.
Born to a family that has thrice defeated the Dark Lord, born in the seventh month, the Dark Lord will mark him as his greatest rival.
But he possesses power the Dark Lord does not understand, and one must die at the hand of the other, for both cannot live; only one can survive.
The one with the power to vanquish the Dark Lord will be born as the seventh month ends."
And just such an ambiguous prophecy had directly and indirectly determined the joys, sorrows, partings, and reunions of so many people.
The prophecy pointed to either Harry or Neville.
Although Dumbledore believed it and guided Harry according to its contents, at the end of the story, it was Neville who slew the Snake Horcrux with the Sword of Gryffindor, allowing Harry to kill Voldemort.
It was genuinely hard to say whom the Dark Lord had truly marked.
If that prophecy was not convincing enough, then every intentional or unintentional prediction Sybill made in the books came true without exception, only in ways no one could have imagined.
For example: around Easter, one of our number will leave us forever. That foretold Hermione