Is this grammatically correct: In October, 1347, after decades of trading the most luxurious goods, trading ships sailed into the port of Messina, Sicily, carrying a terrifying cargo--the disease now known as the bubonic plague, the infamous plague in which caused an estimated 25 million Europeans to die, about one-fourth to one-third of the population.

Technically, it's correct. However, it's much too long and convoluted. You should rewrite it as two or more sentences.

In October, 1347, after decades of trading the most luxurious goods, trading ships sailed into the port of Messina, Sicily, carrying a terrifying cargo--the disease now known as the bubonic plague. This infamous plague caused an estimated 25 million Europeans to die, about one-fourth to one-third of the population.

That's much better.

Yes, the sentence you provided is grammatically correct. It is a complex sentence that effectively describes the arrival of trading ships carrying the bubonic plague to the port of Messina, Sicily in October 1347. The sentence also adds additional information about the impact of the plague, stating that it caused an estimated 25 million Europeans to die, which accounted for about one-fourth to one-third of the population.

Here's a breakdown of the sentence structure:

1. The main clause: "In October, 1347, [...] trading ships sailed into the port of Messina, Sicily, carrying a terrifying cargo."
- This part introduces the main idea of the sentence, that trading ships arrived at the port of Messina carrying a terrifying cargo.

2. The relative clause: "now known as the bubonic plague"
- This clause provides additional information about the terrifying cargo carried by the trading ships.

3. The relative clause: "which caused an estimated 25 million Europeans to die, about one-fourth to one-third of the population"
- This clause further elaborates on the impact of the bubonic plague, stating the estimated number of deaths it caused and the proportion of the population affected.

Overall, the sentence is clear and grammatically correct, effectively conveying information about the bubonic plague's arrival and devastating consequences.