Bellatrix Black had not always been mad.
Well, perhaps she considered so by her family and all those within "polite society", but to the rest of the world, she was perfectly sane. Perhaps she was considered reckless, shameless even, but still perfectly sane.
She was just going through a stage of teen rebellion. She was a girl with a spirit which would not be broken, she did as she wanted and she wanted to live. Bella Black smoked and drank and kissed whoever she fancied, she wasn't even all that concerned about bloody purity.
Bella was the kind of girl you went to when you wanted a good time, and she was always willing to oblige. She smuggled in muggle cigarettes and sipped whiskey from a china tea cup and was perfectly willing to do both while sat at the highest point of the tallest tree on the grounds at three o'clock in the morning.
Bella Black was so full of life; no one could ever rein her in. No one else in the world was so quick with a smile, so uproarious with a laugh. Her fellow pure-blood girls regarded her with scorn and concern, she was not behaving as a young lady should. But Bella didn't care, she tossed her unruly mane of thick back hair and threw back her head, life and happiness bursting from her.
She was the life and soul of every party, the joker and the one that made all the problems go away. So what if your parents where pressuring you to join an evil dictator, so what if you had just got Ts on every test you had ever sat, so what if you were becoming betrothed to your second cousin Siegfried , just have a drink, have a cigarette and laugh right up to the alter. Life could never be bad if you just laughed it all away.
There was no slow turn to the dark side, no dark presence creeping upon her. One day she was Bella Black, the next, she was... different.
No one ever found out entirely what had happened. All professor Slughorn had managed to discover was that Rosier, Avery and Lestrange had been in the common room late one night and a spell had gone wrong. Bella ended up in the hospital wing, when she woke a week later she had no memory of the incident and it was impossible to press charges.
Since the victim had no memory and those involved seemed either unwilling or unable to talk, the only two methods to proceed with were priore incantatum or veritaserum.
Unfortunately, all involved parties came for very old, influential families who refused outright to have their children's wands examined and even if they had permitted veritaserum, it was a rather grey area to pursue.
On the one hand, all the students were of age and had done something that was almost certainly illegal and definitely against the school rules. On the other, they were still only students and no one had a clue what they were actually looking for. There was no charm, hex, jinx or curse in existence capable of producing the effects demonstrated by whatever they had done, and since they didn't know what they wanted to ask, even requesting the use of the truth serum was beyond pointless.
Especially since the victim herself saw no problem.
This was not Bella Black; this was the woman soon to become Bellatrix Lestrange.
This was not the laughing, fun loving girl who cared for nothing but living and laughing, this was the sadistic and cruel woman who was soon to become second in command to the most powerful dark wizard the world has ever seen, the murderer and torturer of nameless numbers of innocents, the woman who laughed as she inflicted the cruciatus.
This was not a woman anymore, this was a monster.
No one ever officially found out what the spell was; all that was known was that a few weeks after the incident all those who had been involved had their memories expertly modified by their own wands, nothing but the actual incantation had been removed. There were only so many students in Slytherin who could've done that.
But that didn't change the fact that Bellatrix was now officially insane. This girl didn't throw back her head and laugh, her eyes twinkling with life and fun, this girl cackled and screeched with cruel, cold mirth. This girl started the Slytherin tradition of torturing younger students. This girl didn't climb the tallest tree on the grounds to laugh down at the world and have a drink but to declaim to the world that the Dark Lord was coming and that all those who were un-pure would be destroyed. This girl didn't make the headmaster chuckle at her ridiculous antics; this girl was the closest Dumbledore ever came to permitting Filch to do as he pleased.
In one year.
Imagine what she would do in the rest of her life.