For Wilhelm's backstory and attributes: Wilhelm the Courageus
Major Evans: Evans is the leader of the Knights of Thrul. He was a strong and powerful man in his youth, but his age has gotten to him. He is very honorable, though such ideals have been worn down by the political game he is force to play. His sentencing of Wilhelm weighs somewhat heavily on him, because he knew that Wilhelm's sentencing was unjust.
Sir Leanard: Leanard is the Knight who spear-headed the group that blamed Wilhelm for the events that ended with Wilhelm's out-casting. Wilhelm hates him and has sworn revenge.
King Richard: He has sat on the throne of Thrul for a while now, and his rule has been mostly fair. He wouldn't stand out in Thrul's history as the best king, and he wouldn't be the worst, either. He has been King for about three decades now, since he was 15. He was the one that was King during the war with Del- now called the Delian War.
King Callius: He is the King of Del. He is still a young king, being appointed after his father, being disgraced with the losing of the previous war with Thrul, stepped down. It has been a decade after the Delian War, and Callius burns with the religious fervor that started the war, but such fervor is tempered with caution. He knows that Del cannot afford a war right now, and to go to war will require a strong reason.
King Othburt: He is the king of another country, called Zania. Othburt would like to expand his borders, but is currently too afraid of all his neighboring countries to actually declare war.
High Priest Arnius: Arnius is the High Priest of the religion devoted to the god Assiliyane. He is getting to be fairly old, and is growing to rely more and more on others. He does not want war, and thinks that the previous Delian war was a bit silly.
The Basic Layout:
A backbone of mountains, with Thrul, Del, and Zania lieing on the same side of the mountains in a line. Del is on the coast to the west of Thrul, and Zania is on the eastern side of Thrul. Miscelanious other countries are south of Thrul, with the mountains to the north (and other barbarian tribes across the mountains in the hill country.
The Plan (Assuming no PC Intervention or that He Knows of the PCs)):
Wilhelm feels that the best way to take revenge is to destroy the kingdom of Thrul. To do this, he feels the best way is war. As such, is overall plan is to get Del and Zania to declare war on Thrul. Togther, they could crush Thrul and Wilhelm would have his revenge. But how to get them to attack?
Knowing that if he can convince Del that they should go to war, they will easily get Zania to go to war with them (or, really, any other suitable country, if the PCs somehow make Zania not a viable option), he has decided to try and motivated King Callius. Wilhelm has since bent his honor slightly (he never really understood the phrase "all's fair in love and war" until know) and bribed a priest under the High Priest Arnius. This priest has since started spreading rumors of a holy relic that had been in Yoran (a city that Thrul had conquered and returned during the Delian War)- a relic now sitting in Thrul's vaults. At the same time, he has been spreading rumors in Del about how weak Thrul is, how their military took a pounding during the Delian War and has not actually been restored to its former might.
This push-and-pull effect would have the desired effect. It would make Del's citizens want war with Thrul, and the citizens themselves would force King Callius into a war that he, in his heart, wanted. Callius himself, even if his citizens didn't, would know that Thrul was his match, if not greater. Del, like any other nation, had its spies. And so Del would no doubt go seek allies quietly. And he would no doubt inquire of King Othburt. And King Othburt would, since he wants to expand, but only needs allies to do so, would accept.
Of course, some power centers on both sides would try to stop this war. And these centers Wilhelm has attacked- along with his bandits at his back as support if required- so that the war effort would go unhindered. After he has done this, he plans on attempting to join either Delian or Zanian forces and actually lead them into battle. As an ex-Knight, his skills are more than apt for the job.
And when the combined armies conquer all of Thrul, he will ride into the capitol city, enter the Knight's barracks and announce his presence to all there (and give the trademark of an evil villain- the monologue). And after that, he personally will duel Sir Leanard, and have his men kill the rest of the Knights. His revenge complete, he will live out the rest of his days in peace, leading either the Delian or Zanians military.
That is, this would happen unless other factors do not come into play. And these other factors are, of course, the PCs. The point at which they find out can occur whenever. Perhaps have them attempt on hand in Thrul and be sent to track down Wilhelm after he escapes- and have Wilhelm die in a way that is positively certain to kill him- except for the fact it didn't. After that, have them discover Wilhelm when he takes out one of the power centers that could have ended the war effort. And then they can unearth the rest of the plot.