I did a quick search for the story on the web and I found a few links that almost support the story as I remember it. Truth to tell, all that I remember is Sir Owain defeated the Black Knight and got lost in the Black Knight's castle searching for The Chalice. Well, that's not quite the way story seems to go (although there are probably several versions, as there often are when it comes to the old legends) but the part I'm focusing on-- Sir Owain confronting the Black Knight-- seems not to be affected by my errant recollections.
...The Chalice doesn't seem to be mentioned anywhere! Oh, well.
...These are my first roughs, and I'm still searching for a good direction. There is another rough I'm working on but it's not quite ready. I have a new take on the Black Knight character which I'm liking and more ideas are flowing from that. Update soon
The End