Why are they fighting? For us to have interest and empathy, we should know why they are fighting. As it is, all we know is two knights are doing battle - and knowing nothing else keeps us at a distance emotionally. I think this opening would be better if you cut down on description and instead introduced some conflict into the story - a reason for them to fight. As it is it's a static descroption of an event. Give us someone to care about, with a reason to care about them, and you'll get us at an emotional level - and you will have a good start on a story.
"...cut it like butter." Cliche
Corny example, just to illustrate my points (you can do much better):
Joe Bob paused to catch his breath, but wary of the Black Knight, who looked ready to charge at any moment. "Cease, Sir Black Knight. Observe the carnage!" He indicated the battlefield; the full moon illuminated the remains of people and creatures lay scattered at their feet.
"I'll kill you now, Sir Pipsqueak." The Black Knight charge.
It occurs to be you may be going for an objective "fixed-camera" POV, perhaps as an intro section. That is notoriously difficult to pull off.
Unless there is some aspect of someone's appearance or clothing that will soon be relevant to the story, you can usually omit these descriptions. Just mentioning a tall Black Knight and a short Silver Knight gives me enough information to picture the scene. Unless the rubies on the sword are going to pop up and do something amazing in the next paragraph, I don't really care about them. It bogs down the pace of the story, especially at the beginning.
The hook is good. Two knights fighting alone amidst scattered carnage makes me wonder what's going on.
The mixed past and present tense has already been mentioned, but it's still true that after all the past tense description ("stood", "was", "had", "could" [instead of "can"], "were" and "lay"), the sentence "The Black Knight charges the Silver Knight" comes as something of a jolt.
In the sentence "He was in all black armor and a helmet with a long thick sword that had two red rubies in the hilt," my first thought was that the helmet had a sword. I had to reread it to get it.
Try taking out some description, or moving it to later on in the story, or better yet, attaching bits of it to actions: "The [hulking] Black Knight charged the Silver Knight[, swinging his broadsword towards the slim neck of his opponent]. This is just a quick example, but I think it reads better than writing out the description and then leaving the actions bare and un-adjectivized.