Trademark Domain Name - they want it back. What Do I Do?

Basically, nothing. You show you're using the domain name to identify the source
of your goods or services, although how to do that can be challenging.

That's where filing an application that hopefully will be granted TM status comes
in. Ask your local government office how much or so.

You know, one competent lawyer I know once told me you generally don't have
to agree to things another wrote for you. While you can transfer the domain to
them, you don't have to do that item I quoted above.

This is a suggestion and not advice, but tell them you can't realistically agree
to that unless, say, they provide you a list of all their trademarks. If they are
unable to do that, it's going to be a big question how they'll be able to legally
enforce that.

But if you already agreed to that, then never mind.