Dude should start thinking politically.
Rigar is an obvious hothead and is way too ambitious. Study the law of the land and how best to use that to make Rigar the offending party. Then it's as easy as to feign weakness to Rigar. Rigar, the overly ambitious, will try to exploit it. Lead him through tactical withdrawals to the most politically compromised situation you can, and then stand your ground. Rigar will most likely see it as a hurdle and not a wall and will try to clear it. This will bring him outside the law in a way that's usually overlooked in victory. Then you press back and use this legitimate cause to gain the support of those you can, but most importantly the abstention of those that might support him. Then it is a matter of removing him from power as swiftly as possible.
As for a possible plan:
It is obvious he does not realize Almus' personal power nor does he see him as a strong rival. Rigel is also Almus' direct neighbor which provides a lot of opportunities. First, Almus should make sure that Rigel views him as a coward, pushover and physically weak. An easy way would be to have a bad actor deliver the 'real' story of Almus' victory, showing a boy led by his generals, fearful in battle and saved by his bodyguards on the battlefield. Rigel seems the type to believe what he wants more than what is most evident, so he should go for it. He's also an opportunist and scummy so he should see a chance to gain from this weak neighbor of his. Almus should concede to whatever demands Rigel makes, goading him into making bolder claims. All the while playing the coward. At this point it would be good to start planting the idea of a duel in Rigel's mind. The fear of combat in the false story should hopefully be enough for an opportunist like Rigel, but that idea should be nurtured through carefully chosen phrases and, best of all, by putting up false blocks for Rigel and having them completely collapse at any hint of violence, best of a clear duel. This should goad Rigel into making bolder and bolder claims. Depending on the political climate, an official protest might be a good idea but, if the country is centralized enough, then spreading the news of what is happening there instead of risking government action is the better course.
What we want from this phase of this plan is:
- Rigel to be seen as a villain. the aggressor and the offending party. Easy peasy. That's what he is, but he gets away with it because he's a successful villain.
- Rigel to make and act on claims that are not legally enforceable, but can be made to work with a 'might makes right' method.
- Rigel to perceive any hesistance on Almus' behalf as token resistance.
- Rigel to see a duel as the ace up his sleeve and Almus as a weak opponent.
In phase 2, Almus should stand his ground on a matter. It should be something significant. Best if it is politically significant. This should force Rigel to pull out the 'duel' card, which Almus should accept as hesistantly as possible. The object here is to not show your hand and have Rigel continue to believe that he has the upper hand and that this will soon turn in his favor. Now I have no idea what role duels have in that country, but Almus should link as much to the outcome as he can right up to betting all his land for his. If duels are strictly considered personal, then that is also fine. During this phase Almus should have elements ready to capitalize on the chaos that is about to unfold. That means:
- military units in locations or state that will not be threatening to Rigel, but also capable of quickly taking control of Rigel's territory.
- envoys to other powerful families in the kingdom to get them to abstain from any involvement and convince them you are justified in your actions.
- political agents to gain legitimacy from the government for whatever action you take.
- diplomats in Rigel's region to pressure whatever command structure exists there with threats of violence and reparations to force obeisance.
Now we come to the duel. If all goes according to plan, Rigel shouldn't suspect anything. Now the matter is as simple as this: You kill Rigel. I don't know how duels work there, so it might be capped at first blood, but there's no way to remove the risk of death from duels and so it should be completely doable to kill him. Best would be for Rigel to think he's on top up to the very point he sees the spear sticking out of his neck. by removing Rigel, you remove the main danger of his clan, but you also completely rock the power structure in his territory. This will make it easy to gain control over it before they can get their act together. The key point at this stage is to decide whether you want o annex it, or position a puppet as the leader of Rigel's clan. The puppet is the better option. There should be reasonable elements within Rigel's clan that can help the kingdom. Almus also doesn't want to be seen as power-hungry, even though justified, and so it would go a long way to use reparations as a threat, but to not act on them.
