How to handle A Campaign based on A Film?


I'm working on A RPG Campaign that's part 3.5 Edition DAD and part Legends Hybrid and want ideas for the main bad guys force for the final showdown in a big battle followed by A Sneak Attack via Magic to take out the campaign's ultimate evil and its based on The Dragonheart Film from 20 odd years ago

The good guys can muster around 436 rank and file who are 3rd to 5th level and 69 regular officers who are 5th to 9th level and will be derived from every official class, their leaders are A 13th Paladin who served the king before he showed his true colours and spend 12 years repenting to regain his Paladin Status, A 12th level Cleric whose The Kingdoms last Cleric of higher then 8th level whose alignments not at best Neutral-Evil and for the most part Evil or Chaotic, The Daughter of the now dead former resistance leader whose A 13th level Thief, the current resistance leader whose A 13th level Fighter and A Very Ancient Half-Copper and Half-Red Dragon who casts Spells as A Ch 27 and Int 20 15h level Sorcerer

The idea being that it could go either way. With the evil king having the edge of numbers, quantity on weapons and a willingness to use Poison, Black Magic, Summoned Deamons, Cannon-Fodder Monsters bribed with Food, Loot and Enemy Prisoners as their choice of Food, Slaves and sacrifices and Undead and the good guys having the edge of quality of troops, quality of weapons, quality and quantity of training, experance and loyalty to the cause and their lowest level Leader has the same number f levels as The Enemies Leader and King

The idea behind how the final battle pans out is that if the characters aide 1 side that side is guaranteed to win the main battle and have a very good chance if winning the war, but if they don't I'll leave it to chance and hope the characters don't have to fave the wrath of the winners

But the King/The Campaigns greatest being allied to Chaotic Evil whose Race is Pureblood Human, no push-over cos due to his abilities he has to be at least A 6th level Fighter, A 3rd level Assassin and A 3rd level Fallen Paladin turned Blackguard who has been made also super-human thanks to Deamonic power granted for the sacrifice of 20,000 odd souls and as his old teacher was, prior to this guy showing his true colours, once regarded as The Greatest Paladin in at least 12.9 decades it makes sense for him/The King, to have more Skill Points then normal in some Skills that are normally Class Skills for Paladins, but naturally not those that can reasonably considered to be due to A/The Paladins Holy Nature
Difficult to give you ideas for Legend because you describe everything in D&D terms, which I don't play.

What is it your wanting ideas for?