I'm so sorry Winter.
This must feel hellish. You've just found out though and what seems incomprehensibly difficult to contemplate at this moment, might not seem so frightening as time moves on.
I know, because I felt very much like this about my child's father having contact with him, invading our lives with his poor behaviour and being effectively a threat to our happiness for life. Five years on and he did leave us alone and I can now cope with seeing him, with knowing he knows where we live, with having a chat now and again. He still is not part of our lives but I am no longer terrified of him.
I think my point is that you have raised this person into a spectre so terrifying that you can't cope with her. Or her child.
She is probably not that bad. Almost certainly her child will not be that bad. What is that bad is your fear, and that's understandable and natural. It's greater than the reality of it. Right now, that's normal and it's Ok. It will lessen and you will cope with whatever you need, or choose to cope with.
What I am getting very strongly is the sense that in order to stay with a husband who has made such incredibly poor and selfish choices you have had to shove all of that trauma over onto the OW, so you have got him in your life, that was your choice for your own reasons and you are no longer afraid of him, or what he did. You're not furious with him, or scared of him but instead you're furious with the OW and very scared of her.
The fact that she might now be someone you have to let back IN, while until now you could keep her OUT of your lives, along with all the horrible feelings she represents, has hit you like a truck and stopped you being able to keep all those bad things away and out of the picture, so that compromises your efforts to bring things back to normal with your husband, because it dumps all the fear and anger right back in the picture where it started.
This is extremely unfair. However it is true that your husband caused this. He brought her into your lives. He chose to do this with his lack of care and reckless sexual behaviour. And now you've got no choice but to face that.
I am so sorry this has happened. First of all I'd be looking to establish whether or not she is pregnant, and I'd also be asking him to move out. That way he can speak to her and sort this mess out without involving you. You get your life back, untouched by her or a new child and he sorts out the mess he made himself.
And then once it is sorted out, one way or another, he can come back, or he can't, but he has to leave her outside, as it were...he must take care of the child's interests and to some degree, hers too. But not to the detriment of your own family together.
If you stay it will be really hard. But it will get easier once you start to feel you have some control again, and I think at the moment that's the worst thing, because you feel as though you have none.