Champion NPCs are the "elite" unit of any given faction. They are meant to shake things up for a more experienced warrior of Middle-Earth, with new strategies for winning and new quests.
Champions will only spawn at 500 or -500 alignment with any different faction. At 500, the champion units begin spawning in as ally units in the faction borders. At -500, they spawn in as hostiles. They will not spawn in invasions, constested areas(Ithilien spawns no champions), and unpopulated areas. In multiplayer, champions will not spawn unless all the players in a chunk meet the requirements. Peaceful Factions (Hobbits, Dorwinion) do not spawn champions.
Champions will mostly look like the regular faction soldiers, with a few key differences. They wear no helmets (only aesthethically, they still keep defense bonus), and hold a modified shield. (Gold trimmed shield for good factions, cracked or glowing for evil.) If there is an "elite" faction armor (gold-trimmed Dwarven armor), they will also wear that.
Ally champions will act as regular faction units. Their quests are more geared towards combat and war, with no fetch quests. Champion quests may involve hunting down enemy champions, fighting bosses, or even killing enemy traders. (There should be an option to disable this particular quest in multiplayer to avoid base griefing.) Occasionally, there will be a sparring quest, which involves duelling the ally champion. The champion cannot be killed, and surrenders when at low health. Likewise, they will stop attacking you at 3 hearts or less. (More on this in combat)
Enemy champions will not attack you until there are no more units fighting you, to avoid unfortunate incidents where a champion sneakily murders you while fighting a horde of enemies.
All enemy champions have a certain gimmick to encapsulate their faction's fighting styles.
Gondorian/Rohan Champions summon 1-health ally units to attack you, to show off their "leader" characteristic.
Orcs/Uruks drink Orc Draughts when entering combat, and try to escape at lower health.
Rangers try to stay back and snipe you, drinking Athelas brew to heal when at low health. (1-time.)
Elves use Miruvor, and disappear and reappear similar to the Mirkwood Scouts.
Haradrim, Southrons, and Corsairs will automatically use Poisoned weapons and Firebombs while fighting.
Completing a champion quest and murdering a Champion both give the same rewards. A "high quality" weapon or armor piece that has 1-2 prefixes on it, a large faction bonus or decrease, and some money.
Q: Don't we already have elite units in the form or Swan Knights, Shieldmaidens, Etc.?
A: Most elite units here are more of elite "organizations", rather than powerful units. Once you master how to defeat a Swan Knight, the same principle applies to all swan knights. Champions are intended to give experiences warriors more of a challenge.
Q: Won't this be pretty hard to code in?
A: Many of the ideas here already appear in other parts of the mod. Hobbit Shirrif Parties appear at a lower faction count. Many Orcs already run away when a certain trigger is met. Units already drink or eat when at low HP.
Q: Can the Champions be recruited?
A: No. Either the recruitment is simple, and a player can eventually have a champion army, or it's too difficult for anyone to complete without sacrifices. The champions are intended to be your "equal" in some ways, not your subordinates.
Thanks for reading this!