I've already done a little of this. My son has asked about my mother, her name, where she lives.
"Her name is X. She lives here in Portland."
He has asked if we will see her. I have told him that, no, we will not. Gently, emotionally-neutral voice. Why?
"Well, Kiddo, because she's not a nice person and she's not a safe person. So we don't visit her and that's okay. You have Gran and Grandpa, and Mae Mae and Papa who all love you very much." (They are my husband's folks and my bio-dad and his wife, respectively. He has good relationships with all of them.)
I imagine that when the questions become more specific, I will try to help him understand things on a child's level. "You know how some kids aren't kind with their words (or their bodies-- when he's much, much older)? Or how some kids hurt other kids on purpose? That's what my mother does. So we don't see her because she just doesn't know how to be nice."
I will not go into any specifics until he is much, much older. He told me once that he wanted to meet her. I told him that he could choose to do that when he was a grown-up, smiled at him, hugged and kissed him. I want him to know that his desire to know her isn't bad and isn't going to hurt me. When he's older and has a better capacity for understanding, I can tell him more, because I do want very much for him to guard his heart. She was more than a lot of people could handle, even many adults. So, when the time comes, I'll be honest with him, but for right now, we are focusing on the family (blood relatives and good supportive friends) that he does have.