First off: a well thought off campain, good story and properly prepaired
Think about this, if the king has a super secret plan, whould he work so out in the open?
I'd take an other approch, but it's depending on how long your campain is.
1. The characters are making their way towards the city, on their way they see a merchant being robbed, they help him. (critical point, make sure you have a back up!)
2. The merchant asks them to come with him to his house in the city where he rewards them. He is looking for an escorte to other cities as he is making an trip there that may not fail. (he is the official messager to get reinforcements for the king) He will pay them very well for it, most of it afterwards.
3. As they travel to the other cities, the mercahant meets with the ruler in the first city, the others don't know what he's doing. They don't even know he's meeting the ruler himself.
4. On their way to the second city, they are attacked. The merchant is killed but the players kill the attackers.
5. In the merchant's stuff, they find a letter from the city's ruler with the assignment for the merchant. The merchant had a secret contract with him for many years, especially for assignments like this that are to be kept secret.
6. They players furfill the assignment. Motivation could be an extra letter from the merchant to them in case of his death. But only if the first letter isn't enough motivation. Plus make sure they were too get paid when the got back! Money always motivates.
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