If anything, the game needs be to made harder at the end to give the player a challenge and something to do once he has all his honor troops and so forth. I suggest the following, how about [Villain Name] The Brainwasher with his army of 400 Noldor Twilight Knights that you have to fight without help from your lords? It can be scripted as a quest so that this army doesn't attack and isn't attacked by anyone else like the Jatu Army Quest. That will prevent the army from becoming too large and impossible. A scripted dialogue could explain the lords' reaction. Lord X: Umm, I just remembered that I promised my wife I would spend more time with her. She'll kill me if I don't. I'll be back soon...". "Uh, yeah, you know I just got this message from one of my village elders, there's an emergency that I have to take care of right away". The player is expected to fight this army several times and destroy it through attrition. This will be fair even for hardcore players because they can just leave their most valuable equipment at one of their castle chests. Finally the player will win and there should be no option to kill [Villain Name] because that will set up the final battle where the player will have to assault a castle stocked by a mix of 600 Twilight Knights, Doomguides and Hero Adventurers or whatever top top level troops that are available in POP 4.0. Again the player will likely have his entire army destroyed several times and then have to rebuild in order to conquer this castle (the ramp should be somewhat wide, not single ladder as to give the player a chance). If possible, the player should have to face the villain in an one on one battle inside the castle without any help from his companions. The villain should have better stats than a Twilight Knight, but not so high that it'll be impossible for the player to win this final battle. That will give an incentive for the player to use Qualis Gems to pump his own stats up and he can use his very best equipment.
Basically this is to remind the player that he is playing Prophecy of Pendor, and no matter how awesome he thinks he is and how powerful he thinks his army is, there is always someone more badass and better in this realm. Not recommended for beginners!