Harry Potter had brought grave news, grave news indeed. There was no need to trouble the child with the nature of what he suspected such words would mean, and he had brushed it off as "nothing of great importance," then spent every waking hour working towards a cure for the inevitable.
But no cure could be found. The Unbreakable Vow was called unbreakable for a reason, and not once had the Vow of a full-grown wizard been broken without death. And so he had asked Severus to meet him, far from the school, to discuss the first misinformation Snape had given him since joining the Order.
The former Potions Master was on time to the second, swooping into the forest warily and looking about for a moment in the dim light before spotting Albus, leaning patiently against a tree with an unreadable expression on his face.
"You sent for me, Headmaster?" said Severus, looking at Albus. The headmaster nodded, and noticed that Snape was not quite confident enough to look him in the eye without endangering the cold, emotionless expression he now wore on his face.
"Harry Potter has informed me of a conversation you had with young Malfoy, Severus, one that you did not inform me of" Albus replied, no hint of accusation in his tone.
"Harry Potter would do well to learn to hold his tongue" snapped Severus, vicious hatred in his voice. But he could never lie well to Albus, even before the Order, and a hint of fear shone through his outburst.
Albus Dumbledore smiled patiently. "That aside, Severus, he informed me that you have made an Unbreakable Vow. Is this true?" he said, although he knew the answer already.
"Of course not" hissed Severus, "I would never do something so foolish, even in protection of the Order." His anger did an excellent job of covering his hesitation, but he could never lie well to Albus.
"So, then, you have taken a vow to end my life, should Draco fail" replied the Headmaster, bowing his head. Snape's sudden silence was enough to tell him that he had guessed correctly. "This complicates matters, Severus, and I must ask something very difficult of you."
"You need not ask, Headmaster, I was prepared to break my Vow the moment I made it" said Severus quickly and sincerely. Albus smiled faintly, moved as ever by the loyalty shown to him by those in the Order. You overestimate me, Severus he thought to himself, raising his head again.
"Severus, that is not what I must ask you to do. When the time comes, I ask that you keep your Vow."
For a moment, no amount of acting skill could hide the dawning horror on Severus' face. "You can't..." he whispered, looking more like the terrified first-year he had once been than the confident Professor he had become. "Albus, you know I can't, I could never-"
"I am dying, Severus" interrupted Albus, stopping Severus before he could continue. He held up his injured arm, flexing charred fingers as best he could. "I am too old for this, the next attack Voldemort makes on me will be my last, and I will not let you, or Draco, fall with me."
"How could you ask me to do such a thing?" demanded Severus, raising his voice. For the most part, when he was angry, his voice would become quieter, and even Albus only rarely saw him furious enough to yell. "Have I not been loyal, done everything you asked of me for the Order? I have risked my life, my very soul in the battle against the Dark Lord, how can you just stand there and take my sacrifices for granted, assume I will make another? I made my choice when I made my Vow. I can't do this anymore, Albus, not if-"
"If my request seems like something Voldemort would ask of you" said Albus, finishing Severus' sentence for him. Snape opened his mouth to object, but at a look from Dumbledore he closed it again- there was no use objecting, he could never lie well to Albus. "Severus, this is not an order, I would never order you to do something so terrible. This is my last wish, Severus, and I ask it of you as a friend. Please, Severus..."
Severus faltered; he had never heard Albus plead with anyone before, least of all him. Something inside him collapsed, and he felt himself wishing he could just curl up and hide from the world like he had as a child. Terrified and feeling very much alone, he spoke again. "I... I will do as you ask, Headmaster."
Albus Dumbledore smiled, for he knew those words meant more than any Unbreakable Vow.