tmn111

“1. His death marked the end of an era.”

There have been many eras. His death ended just one of them.

tmn111

“

2. The circumstances of his death - which was to usher in an era of renewed upheaval across the Empire - were less than dignified.

”

What follows of describes the era; renewed upheaval across Europe is adjectival. It is normal in English for adjectives to bring on the indefinite article:

He had dinner. | He had an early dinner.

Birds were flying in the sky. | Birds were flying in a blue sky.

tmn111

“

He was stabbed by one of his own guards while urinating at a roadside, pheraps perhaps a fitting end for a man who brought such humiliation upon his office.

”

A relative clause can bring on the defintite or indefinite article. In this case the relative clause describes the man and therefore a is used.

CB

tmn111

“

So does it mean that it's possible to use both the and a/an before a relative clause?

If I said: "...the man who brought such humiliation upon his office" instead of "...a man who brought...", would the meaning remain the same?

”

No no! The indefinite article is needed for your sentence. Here's ann example of a definite article brought on by a relative clause:

This is the car [that/which] I bought last week. (I bought only one car last week.)

This is a car I bought last week. (I bought at least two cars last week.)

CB