In a scathing Tumblr post Sunday, the pop superstar writes she is sad and grossed out that her music catalog now belongs to Braun whom she accuses of subjecting her to years of incessant and manipulative bullying, referencing clashes with Kim Kardashian and Kanye West.
"This is my worst case scenario," Swift writes.
Braun's Ithaca Holdings announced Sunday that it is acquiring Big Machine Label Group which released all of Swift's studio albums and owns her masters. Swift says she learned of the sale Sunday.
Swift left Big Machine and signed with Universal Music Group in November because she says she knew that re-signing with the group that had managed her since she was 15 would only result in her not owning her future work.
"When I left my masters in (Big Machine Label Group founder Scott Borchetta's) hands, I made peace with the fact that eventually, he would sell them," she writes. "Never in my worst nightmares did I imagine the buyer would be Scooter."
Braun, she alleges, got two clients, including Justin Bieber, to bully her online about a leaked and illegally recorded snippet of a phone call she had with Kardashian. She also references when West, a Braun client, organized a "revenge porn music video which strips my body naked."
She writes that now Braun has "stripped her" of her life's work that she "wasn't given an opportunity to buy."
"My musical legacy is about to lie in the hands of someone who tried to dismantle it," she writes.
Representatives for Braun and Borchetta did not immediately respond to request for comment.
Justin Bieber responded by defending Braun, his longtime manager, but also apologizing for one of his own posts about Swift.
"Scooter has had your back since the days you graciously let me open up for you.!" Bieber wrote on Instagram.
"As the years have passed we haven't crossed paths and gotten to communicate our differences, hurts or frustrations. So for you to take it to social media and get people to hate on scooter isn't fair."
Swift ended her own post by looking to the future and says she thankfully "signed to a label that believes I should own anything I create."
She also advocates for artist ownership of songs and hopes that the next generation will "read this and learn about how to better protect themselves in a negotiation."
"I will always be proud of my past work," she added, making sure to also plug her new album, "Lover" which will be released on Aug. 23.