A/N, Written before HBP, looked back at it after HBP and decided Snape kept some, ten guesses why and for whom!! I wish.
Disclaimer! I own nothing and nobody! (But the potion is mine...)
The group of High Masters looked at each other in disbelief. Never had they seen this potion brewed. Never had they met any one willing to try, or able to succeed. Oh, there were those among them for whom it was possible, but none there for whom it was a possibility, unsure as to whether they should be impressed or terrified; they exchanged looks that covered both these feelings and every one in between.
There were nervous glances at the door, outside of which stood the dark young man who had spent three days locked in this room, brewing his master piece for the inspection. For three days the waiting men had guessed what it may be. Potions Masters weren't made all that often and they had known this one was coming for a while. They knew it would be something good, but not this. This of all things.
"We can not dispute the ability." said one.
"Nor the fact that this is a true Master Piece." said another.
"I should like to question his reasons," put in a third, "ask why this particular potion. A dead friend perhaps…"
"I am not entirely sure I'd want to know…" quietly added a very nervous voice from the back of the room. "Any Wizard willing to sacrifice a part of his own soul to make a re-birth potion must have a deep reason. More than just a Master piece."
The others contemplated this silently.
"He comes highly recommended"
"Albus Dumbledore supports him entirely."
"So he supported Tom Riddle…" They all shuddered slightly.
"We shall not ask. All those in agreement that this is, indeed, a true Master Piece, and who wish to grant the title 'Master' to the brewer, say 'Aye'." A chorus of 'Ayes' went round the room. Still glancing nervously at the potion in the centre of the room, the door was opened, and the oldest Wizard, having summoned a silver ladle, covered in symbols which seemed to shift if you looked at them, ordered "Enter here! Severus T Snape, Master of Potions, and be seen!" The dark man entered the room, face utterly unreadable, and knelt before the Wizard.
When all the rituals and traditions had been seen too, one of the men, feeling a little braver than the rest in the company of this new Master who exuded confidence and disdain, asked why. The question was considered, before an answer was given which confused him slightly. "For the promise I made. Because I had to."
A/n 2;- Re-birth potion is a potion which can only be brewed by a brewer of immense talent and dedication. It requires him to undergo a painful process to remove part of their soul, similar to a Horcrux, only adding it to the potion not an article. The resulting potion can bring life back to a dead body and/or call back the soul if used soon enough. It can only be used on someone the brewer truly loves, be it a family member, friend or lover. It is nearly Dark magic, although its use it not easily twisted for evil due to the love required. It is, however, a Ministry controlled potion, allowed to be brewed only under supervision or in a Potions Master Piece. The pot brewed by Severus Snape was done so with the full knowledge of Albus Dumbledore, and dully collected by him for the Ministry. What he did not know was that Snape kept a vial, just incase the one true friend and mentor he ever had should one day have need of it, someone who truly loved him could administer it.