What is the best thing to do for my 3 kids after finding out my husband of 13 years has a pregnant istant?

I am convinced he is a narcissistic man. He has lied and cheated consistantly. I have buckled down and done everything to please him and save my marriage. When finally he seemed to be happy he unloads this. I dont want to ruin my kids lives. But I feel he is setting this up to have us both if he can. What is best for them? I have offered to take the baby as our own. Should I make ultimatums? Should I stay and wait it out? I am devestated.