By Virtue, Not Birth by Timothy Underwood

When she was five years old, Elizabeth, the legitimate daughter of the Earl of Rochester, was entrusted by her dying mother to Mr. Bennet’s care.

As she grew up, Elizabeth remembered very little of her childhood from before Longbourn. But she remembered being violently beaten by a man who shouted that she was a bastard as he hit her. Because of this Elizabeth had always believed that she was illegitimate. She lived as a poor relation at Longbourn, deliberately dressing plainly and doing everything Mrs. Bennet asked. Only with Mr. Bennet, surrounded by novels and Greek and Latin texts was Elizabeth able to be her true self.
Mr. Darcy did not know why he was so drawn to the strange, brilliant, penniless girl he met at the Meryton Assembly. But he knew that he hated how Mrs. Bennet treated her. And he knew that he wanted to protect Elizabeth and see her be happy.
What will happen when the truth about Elizabeth’s father is at last revealed…

Note: 105,000 words long, and an event of severe spousal and child abuse is central to the story.