I see this is your first draft of your first novel. Congratulations! And welcome to WDC!
I'm a big believer in making first lines and first paragraphs depict your character, your theme, and your style.
Your first line: "As the bright orb dropped below the jagged line of the horizon, the light which had been provided by the sun was replaced by that given off by flickering flames."
This tells me you're trying to force description, but it says nothing else other than it's dark and there's firelight from some source. Candles? Bonfires? Lighters? Okay, probably not lighters, but you get my point. Bright orb sound convoluted. I'm guessing you mean the sun. Why not say that?
As the bright orb dropped below the jagged line of the horizon, the light which had been provided by the sun was replaced by that given off by flickering flames. As adults in the camp herded the last of the children into tents to put them to sleep, others checked the enclosures for the animals. These were the first to fall, arrows speeding out of the night. The only sounds were muffled gurgles as they sunk to the floor. And then there were horsemen in the centre of the camp, riding in all directions scything down the now terrified tribesmen with long cavalry swords and lances. Men, women and children were all claimed, the darkness made the slaughter indiscriminate as the faceless soldiers rampaged through the camp. As the initial shock passed, people began to grab weapons, and pockets of resistance sprung up around the camp.
I see several issues here that would turn me away if I picked this book up in a store. (And I always read at least the first paragraph before buying.)
"As adults herded.. others checked." This is very distant and cold. Adults? Others? Who are they? Why should I care? "And then there were..." This is also distant and passive.
"These were the first to fall..." I'm confused. These what? Animals? The adults checking the kids? Or those checking the animals.
Overall, I'm very lost at the very beginning and I'm not likely to wander farther because there's nothing to make me care about the nameless, faceless beings wherever they are. You might consider starting with Ingvarr, with his thoughts about what he sees and where he is and who he's with in order to give readers a chance to care about the fight that's about to occur.
As I skim through the rest, I can see that you have well-thought-out lead characters and a complex story formed. With some work on your description, making it natural instead of forced, and more flow (which comes with practice), this will develop nicely.
Best of luck with your burgeoning writing career!