Now that Peter Capaldi has a new job as the 12th Doctor on Doctor Who he no longer has time to play the villainous Cardinal Richelieu. So the Cardinal will be killed off screen as reported by the Radio Times. Actor Tom Burke who plays Athos confirmed this in a quote to the Radio Times.
"Come the next series, the Cardinal will be dead. He'll just be gone. It's a great example of economic storytelling. Their new nemesis will be Rochefort, who's played by Marc Warren."
Source: Digital Spy