Nightshade used to be a pony living in ponyville by the name of Apple Harvest, with a hazel coat, brown mane and green eyes.
One day, the town was having a crisis; a plague spread by flies, they were destroying crops and ponyville was due to run out of food, and ponies were getting sick too! The townsfolk were trying to think of a solution to the problem. Even Fluttershy couldn't communicate with the flies.
She found out about an old artefact that slept within the Everfree forest. The artefact was said to be the staff belonging to a pony who in old times was a kind of druid or wiccan known to practice rituals of protection and bounty through nature, but at the time, ponies were afraid of her magic because they didn't understand it and she went into legend as a witch.
But her staff was said to bring bountiful harvests and harmony with nature. So she set out in secret to look for it.
The young explorer found a grove hidden deep in the forest, shaded from the sun by the thick canopy. At the center was a pedestal lost to time and overgrown in moss, with an old, black twisting staff at its center, illuminated by a faint glow from the pedestal.

When she touched the staff, she felt it thrumb with power, and a strange feeling came over her, and she was surrounded with magic and an intense pressure, before she blacked out.
Back home, Ponyville gradually found itself becoming overrun with spiders. Friendly spiders! They had come from the forest and scuttled around Ponyville, spreading their webbing. Eventually, the population of flies started dropping, until the spiders disappeared as mysteriously as they came, leaving everything back to normal. The friendly bugs even allowed a doctor to extract some venom from them for use in medicine.
The young pony's actions did inadvertently help the town, thus succeeding in her goal. But there was a complication. When she woke up, she found she had been transformed into a spider pony!
The staff had turned her into a drider, a part pony, part arachnid, and used her power to command the spiders of the forest to save the town. She wouldn't know about this until later though.
The poor lost drider was too afraid to go home, out of a fear of what her friends and family would say when they saw her. She was alone and confused, and took on a new name; nightshade, which she felt fit her new form better.

She got used to her new form though, even grew to loe it! She could jump through the trees and climb very easily, and her web allowed her to swing around, which was very fun.
Over time she grew lonely though, the spiders of the forest weren't much company, and now she dreams of the day she might make some pony friends again.