The answer to your qustion would be that the complete sentence that appears on line 18 of the King Arthur's Knights excerpt is an example of a complex sentence.
A complex sentence contains an independent clause and at least one dependent clause. In this case, the dependent clause is "while it was yet dark" and the independent one is "a great strange cry rang out in the churchyard."
He uses this secondary purpose to help support the primary purpose: to persuade readers that a conquering prince must destroy a former republic if he hopes to hold it.
