"Will you, Severus, watch over my son Draco as he attempts to fulfil the Dark Lord's wishes?"
"I will", said Snape.
"And will you, to the best of your ability, protect him from harm?"
"I will," said Snape.
"And, should it prove necessary... if it seems Draco will fail, will you carry out the deed that the Dark Lord has ordered Draco to perform?"
"I will," said Snape.
