First, welcome to the forum. I hope we here may be of some help to you.
Second, no, you're not going to die from fear. Do take some time to breathe, though, and to calm yourself as well as you can before you turn to face the source(s) of your fear.
After all, nothing has actually happened yet, or so I assume. Your husband has told you be wants something, but his wanting it does not in itself impose some obligation on you, nor does it mean a decision has already been made.
In the end, you may end up simply saying "no" to your husband. He may be disappointed in not getting what he wants, but clearly you have something to say about the form your marriage takes. Your husband's happiness is not the only consideration and, besides that, looking out for his happiness in the long run is not the same as giving in every time he wants something.
But you're not at that point, yet.
You really need to talk all this over with your husband, in great detail and with as much candor as you can manage. Some questions to consider:
1. What is your marriage like, from the inside? Is it strong and happy? Contentious and stressful? Something else? Is it possible that your husband actually wants out?
2. What is it your husband actually wants? "Open marriage" is vague. Does he want to be able to have sex with others? Committed relationships with others (polyamory)? Does he want to find someone to "add" to the marriage, forming a triad? Something else? Would you be free to pursue other relationships, if you wanted to? Including relationships with other men?
(If you both agree to go in one of these directions, there are many, many other decisions to make and boundaries to set. This will require lots and lots of communication!)
3. Did your husband introduce the idea of an "open marriage" as a speculative possibility? A request? A demand? An ultimatum?
Really, you just need to be clear and direct about your own choices and limits, and be ready to speak up for yourself. (Not that doing so can't be scary in itself!)
Best of luck with this. Let us know how things develop.