Clue: Ultimate demagogue that will usher in day of reckoning (4,5)
Answer: LAST TRUMP
Explanation: Definition: that will usher in day of reckoning
LAST ("ultimate") + TRUMP (the ultimate "demagogue") 
In the Bible in Paul's first letter to the Corinthians, *"We shall all be changed, in a moment, in the twinkling of an eye, at the last trump*."