Twilight's biological mother.
The last surviving heir of the noble house Atropa by the time of Nightmare Invictus' reign, Belladonna was a magically gifted mare with a talent for crafting potions and other concoctions, with a tongue as quick and poisonous as the venoms she brewed. One of Wheatley's closest friends and confidants during the years leading to his fall, she was entrusted with a lot of secrets surrounding his fears for the future, and paranoid delusions that his siblings conspiring against him.
Belladonna reluctantly agreed to bear the de-facto king a foal, despite her concerns for Wheatley's mental state and his avoidance in supplying her with satisfactory answers. Twilight's conception and the ensuring pregnancy was kept secret, Belladonna withdrawing from the public during the later months to avoid notice.
The first time she saw Twilight was also the last, and the final time she saw Wheatley prior to him becoming the Nightmare; the baby alicorn had been whisked away to be raised solely by her father mere hours after she was born.
The resulting years were tumultuous for the unicorn, as she fled Canterlot due to rumours that the Nightmare was hunting down ponies that were close to him in the past, hiding in a tiny swamp community named Village En Marais. Belladonna died a few months before Nightmare Invictus' defeat and subsequent exile when he found her hiding place, poisoning their tea with the plant she was named for in an attempt to take down the tyrant, save Empyrea, and avenge the daughter she never knew.
From within his daze, imprisoned in his own mind by the being that called itself the Announcer, Wheatley truly appreciated the gesture and wished that it could've been successful, and mourned her loss.
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True black-coated ponies are extremely rare, with deeper shades usually just being dark grey, brown, purple, or blue, shades of black usually only being present in piebald/painted coat patterns (such as Asriel Elafris, the God of Art) — the Atropa house was famous for most of its members sharing the uniform, inky black coat.