Brooks Mortos was born in Canterlot. Back when most ponies used to be alicorns. (Ya know before they were mostly all wiped out.) She was on oddly colored pony. Black wings with white tips, a black mane with white streaks, a white horn with a black tip, black hooves, and an entirely white coat. Not exactly the normal mostly solid color type of pony.
One day her and her parents went to visit Brooks' granny, Cherry Bloom. She was an old worn-out alicorn. Ready to go at any given moment. Brooks walked up to her and stared at her for a few moments and said, "She's dead."
Her parents were in denile of the fact and argued saying that Granma Cherry Bloom was just sleeping. Then they decided to try and wake her up. No luck. They had finally accepted the fact that Brooks had been right in saying that Cherry Bloom was dead. Most thought it was just a coincedence. It was far from such.
Soon after her visit with Granny Cherry Bloom, she had went with her parents to visit with more people. Time after time she predicted deaths and all predictions were true. Soon enough, a skull with a scythe had shown up on her flank.
Her parents were mortified with this. How could a pony have a talent like death? Well apparently predicting deaths is a talent, and should not be taken lightly. At first her parents thought it was just a phony mark. But you can't have a fake mark unless you have eaten Heart's Desire, and nopony knew where to find such a plant.
Because of her obscure cutie mark. Ponies bagan to shun her because they thought she was a murderer.
She went into hiding, plotting her revenge on the ignorant foals that judged her so harshly.
One day, she decided she was going to show them what murder was. So she grabbed her hourglass necklace, and found a scythe.
In about two months she had wiped out all but two of the alicorns. The princesses, Celestia, and Luna. Together they stopped Brooks Mortos, and finally decided the best way to punish the misunderstood murderer.