In my novel, I have a part where there is a war scene, and I need to explain it precisely from the king's point of view. How can I explain the war graphics vividly?
For starters, avoid getting into details.
This is true generally, and even more so during battle. People are moving quickly! It is no time for details.
I've read a battle-scene where the author described specific attacks, and the impression I had was that the fighting was happening in slow motion. With classical music in the background. Really slow music.
The best battle-scene I remember reading included almost no descriptions of the actual battle, but of colors, emotions and cries. Needless to say, this scene left a powerful impression, and I actually felt like I was there.
Of course, since you're writing this from the king's perspective - and the king is normally in the back lines - you'll have to get into overall detail of what's happening on the field. After all, the king needs to know what's going on!
See also: Good action scenes
I'm just an amateur writer, but a seasoned soldier. If I were to write a combat or battle scene, I think I would probably try to describe the calamity of battle.
Confusion sets in very quickly when you lose the initiative in a fight. At that point, there is a good deal of sensory overload. Training kicks in and fighting become an instinctive struggle for pure survival. Extremely seasoned warriors can delay this outcome for longer than a 'greener' force.
So, if you have a well-trained military, with several campaign ribbons on its battle-standard, you might consider a very lucid and methodical description of the general mechanics of the fight. Leave details to the imagination, except where they serve the plot.
If your force is inexperienced, I would use the opportunity to capture as many non-combat related details as possible during the fight, to heighten the reader's feeling of confusion in the fog of war.
How do you explain anything vividly? Observe with all your senses, and add emotions and thoughts. Do the research.
I will express hope that you have not personally been in a war scene, so you would have to find some other way of observing, or use your imagination. You could watch combat footage or news reports of war, you could interview veterans, you could read war memoirs, or you could read other fictional books with war scenes.
Then don't just describe the parry-thrust-advance of swordwork, but how it feels to swing the sword — how it hits his opponent, the shock that comes back up the king's arm (or doesn't), the smell of perforated bowels, the smears of blood and brains, the terrible screaming of dying men. Maybe find some Society for Creative Anachronisms chapter and talk to the folks there about how sword-fighting works. (Substitute whatever weaponry or tech is appropriate for your setting, of course.)
Gritty details go a long way, so I would recommend not dwelling on them too much. I feel like it would be more realistic for someone in a battle to be focused on fighting and staying alive, rather than witnessing all the atrocities happening around him.
There's an old adage along the lines of "after the first shot is fired, all battle plans go out the window." If your king is in charge of this battle, at least in some capacity, it would be good to focus on how easily everything breaks down into chaos, both in a specific battle, and in troop positioning/tactics. For example:
Some books that hit these realistic difficulties of war as well as the gritty violence are Black Hawk Down and Red Badge of Courage. Even if they aren't your specific time period, the concepts can be easily adopted.