The Black Cross was the symbol of the Teutonic Order, is that what you might be looking for? The Wikipedia page is named "Black Cross (Teutonic
Order)", the url doesn't word directly but this will get you there:
I had never heard of The Black Cross before I read your Thread. But please don't keep
us in suspense, can you tell us what you've heard of them and how they've impacted society today?
Their symbology on Wikipedia looks familiar to many other religious orders of the time. But maybe you have heard of something that isn't in the text
books and can share some more insight on their order.