Two weeks later Severus Snape stumbled into his bedroom. He had been working late in the lab and had grudgingly acceded to the Headmaster's demand that he get some rest. He pulled off his shoes and strode into the bathroom. As he emerged he was drying his damp hair with a towel. When he finished he dropped the towel in the hamper and turned toward his bed. What he saw surprised him.
There, floating just above his pillow, was a faintly glowing envelope with vaguely familiar and completely unwelcome handwriting. His first impulse was to burn the letter, but he was nearly immediately seized with a sort of morbid curiosity. What in Merlin's name could be so important that James Potter would write to him after he was dead?
Cautiously Severus reached out a hand and grasped the envelope. He half expected his hair to turn pink or to start spouting rhymes, but nothing of that nature happened. The faint glow merely went out and he was left holding a perfectly normal looking envelope. With a sigh Severus examined the letter for any signs of unfriendly magic and, finding none, sat down to read the missive.
Severus, he raised an eyebrow.
I am strongly tempted to call you something else, but if I did you would only burn this instead of reading it, and I very much want you to read it. I fancy that this will affect you far more than any silly nickname.
Severus growled, silly nickname indeed, Potter had not been the one taunted with it. Severus wanted to put the letter down, or better yet burn it to cold ash, but curiosity compelled him to continue. Whatever this was it was sure to be important. It had better be, at any rate.
Two weeks ago a highly momentous occasion came and passed with no comment from you, so I can only conclude that you did not know. Two weeks ago, Snape, your son was born. Severus paled; if he hadn't already been sitting he would have dropped bonelessly to his bed.
The trouble with this is that he was born of my wife and everyone thinks he is my son. Everyone, that is, except Lily and myself. You see, I know all about that night. Lily confessed it to me not long after I'd got back. She told me it was her idea, but it sounds to me like you got her drunk and took advantage of her.
Severus growled again, which was not particularly satisfying, since what he wanted to do was yell and scream and rip James Potter's throat out. He had not gotten her drunk that night. If anything she had gotten him drunk. Well, perhaps not drunk, but he had had enough wine that he wasn't prepared to resist too strenuously when a beautiful woman asked him to her bed.
Besides, James had been dead; at least they had thought so. They found out later that he wasn't, but it was an honest mistake. Bet you loved that, didn't you, Snape. Especially when I came back. Actually he had felt a vague uneasiness, in case James should decide to punish his misdeed.
How do I know he's your son and not mine? Aside from the test potion, I saw him. It is immediately obvious to any careful observer. Needless to say I could not allow the boy that will be considered my heir to look like you, so two weeks ago, while Lily was writing her letter to little Harry, I transfigured him to look more like me and anchored the transfiguration in his bones and blood.
Not even Lily has noticed the change, though she thinks it odd that your son looks so much like me. She is too in love with him to question it though. So you see, Snivellus, not only have I taken your son completely away from you, no one even noticed.
Severus crumpled the letter in his fist and dropped it to the floor. Snatching a cloak and swinging it round his shoulders he stalked down to one of the abandoned classrooms and flung open the door. Angrily he transfigured a few of the broken desks into Potter-like doppelgangers then blasted them until nothing was left but a very fine dust.
Suddenly a thought insinuated itself into is head. James, while a good enough charms student, had never been brilliant. A charm to send a letter seventeen years into the future was far too complex for him. Therefore this had to be a prank. Only Potter could be brazen enough to try something of this magnitude.
How then, had he found out about that night? He hadn't told anyone and he would have bet a hand Lily wouldn't tell anyone but James. A lucky guess, then. Potter didn't really know, he was just mouthing off.
With a wicked smile Severus evanesco'd the mess he had made and returned to his bedchamber. He knew how to deal with a prankster. Just let the little brat wait until he got back to school.
Just as he was drifting off to sleep a small voice whispered that there was a hole in his logic, somewhere. The voice, however, was throttled in an instant; Severus was very good at believing what he wanted to believe.