Calamity

She was created by natural forces due to Equestria being thrown off balance, which was caused from the First Alicorn Order being kidnapped by Lavan.
She and Erebus were rivals who fought to also gain Catrina's amulet, the one Discord had stolen from her, and the one that Tirek gained from his 'brother', although he just found it under Canterlot before being thrown into Tartarus. Only then was the amulet given to Scorpan by Star Swirl, as a symbol of brotherhood and strength.

Calamity was a creature that nobody on the island -those who inhabited at the time- wanted to be near or accidentally provoke, as she could control the weather much like Erebus, creating terrible natural disasters, and with their power combined in combat against each other, it was never anything you'd want to get in the way of.

Everyone eventually fled the Realm of Darkness to Equestria, which resulted in legends about her, tales which had made their way to Discord, so he knew quite a bit about some 'tall tales' of her before he found her, but he had never actually seen her until he brought her in.

Celestia and Luna eventually used the Elements of Harmony upon her, which was the cause of her banishment to the Chaos realm and her memory loss. They agreed to never mention it to anyone, however, Celestia allowed Star Swirl to keep a secret record in the archives.

Eventually, many years after, Discord finds her floating in Chaosville unconscious. He decides to just take her in and eventually taught her to control her magic. After introducing themselves and finding out who she is, he only teaches her to control her magic because, as much as he hated to admit it, he was terrified of what she was capable of

She's a student at the School of Friendship, and really only cares about the friendship history class. She ends up finding out about her history, as does everyone else, causing the new Elements of Unity to be used on her, with the magic of Catrina's amulet, in which she is concealed.

A few years later, Tirek once again escapes from the deepest part of Tartarus, only to find the amulet had been locked away there as well. He uses it, not realizing that the magic isn't from the amulet itself, but Calamity, and ends up releasing her from it. From there, they form a team-up, and take over the Realm of Darkness.

Calamity is serious sometimes, but she can be fun-loving. She tends to be on the more serious side when it comes to studying and research, but when it comes to traveling and her friends, it's like she's an entirely different draconequus.

However, after her concealment inside Catrina's amulet, she grew to be much more aggressive and cold towards everyone.

Calamity's structure is slender and flexible. Her different parts consist of creatures on the island that merged to her:

-Bull [head, body, & horns]
-Zebra [leg]
-Pony [leg]
-Vulture [wings]
-Rose-ringed Parakeet [arm]
-Cougar [arm]
-Possum [tail]

Despite having wings, she can't fly. Her wing muscles are extremely weak, and the only way she can lift herself off the ground is by levitating, using magic.

Before spending time in Equestria, her biggest key moment was stealing the crown of Lavan. Although it wasn't an actual crown, it held huge amounts of power.
 

Her biggest key moment after the fact was graduating from the School for Gifted Unicorns. She had enrolled believing she always had something to prove to everyone. She didn't have many friends while attending, but she promised herself to never give up and to keep moving forward, because she still had a group of friends who cared about her, including Discord.