By the Rule of Two, the Sith Order is strengthened when the Master has taught all he can and the Apprentice has surpassed the Master. It is the Apprentice's duty, then, to challenge the Master in a duel to the death as the final trial: the Master is to fight with his utmost to test the Apprentice's readiness to replace him, and if the Apprentice is indeed ready, the Master should submit to lay down his life for the Sith Order.

What happens if the Apprentice fails to defeat the Master? Is the Master duty-bound to kill the Apprentice as a duel to the death requires? Why? What if, as almost happened to Darth Bane at the very beginning, there's not enough time to raise a new Apprentice to full potential? Would sparing the Apprentice (albeit with some major punishment) be detrimental to the Sith or is it a case of "better than nothing"?