"An unnatural plague has swept the province killing thousands. In the decaying corpses magic death gems were found. Rumors of the mythical Crown of Darkness have circulated in the province ever since."
I thought perhaps it had created a new site, so I did Dark Knowledge on the province again, but no luck. So what does it mean about the Crown of Darkness?
The Crown of Darkness is a powerful death site. However, the events text is mostly flavor, as far as I know, implying that the Crown of Darkness COULD exist at the province, not that it does. The event does not imply that the Crown of Darkness is actually at the province (or even in the game, for that matter).
The reason for the flavor text is that the Crown of Darkness spreads disease at 10% or 20% (any unit in the province has that chance of getting disease for every turn it stays there). It also incidentally grants a 40% thaumaturgy bonus, so it's very good for nations with lots of death and astral magic.