¡Hola a todos! Tengo esta idea para una campaña sobre ninjas vs samurais. Necesito tu consejo en algunas cosas. 

Aquí está la historia:

Los aventureros comienzan en una ciudad que está llena de samurais y luego se arrepienten y descubren que hay un grupo de terroristas que atacan la ciudad.

aquí es donde necesito consejo porque necesito un desarrollo de la historia en la ciudad y necesito que al final terminen arrestados 

después de eso, los samurais tratarán de ejecutar a los aventureros, pero cuando casi los van a ejecutar, los terroristas los rescatan y luego descubren que los terroristas eran los ninjas y explican que el pueblo de los samurais era originalmente su pueblo y quieren para recuperarlo, entonces los aventureros tienen que matar al rey samurai.

por favor deja tus consejos

I mean, it seems like getting your group arrested could be pretty easy. A city run in such a hierarchical manner could reasonably have any number of codes or laws that outsiders may not be privy to. The question is, how much time do you want to give your group to learn about things in the city or develop feelings for either the terrorist ninja faction or the samurai before they get arrested, nearly executed, and then the tables are turned when the ninja try to recruit them. 

Sorry about this.  My Spanish is dodgy.  

If you're trying to showcase a new location for your characters, nothing beats a missing persons case.  They take people from one end of a city to the other and introduce them to all levels of society, high and low, and different social factions.  Someone can hire your characters to find a missing girl.   The clues point to the ninjas at first.  But actually, a powerful samurai is involved.   He sends the police to harass the players and he has them arrested once they get too close to the truth.  This way, your players feel like they have unfinished business with the regime and want to stay involved even after their rescue.

Si desea destacar una ubicación, la mejor manera es un caso de personas desaparecidas.  El detective va de un lugar al otro y encuentra todos los niveles de sociedad.  Alguien contrata con los characters que hallan una chica desaparecida.  Sospechan los ninjas.  En realidad, un samurai poderoso esta involucrado.  Envía a la policía a acosar a los jugadores durante su investigacion y los arresta cuando se acercan demasiado a la verdad.  Así, el grupo quiere vengarse de el regimen después del rescate.

thanks guys for your advices they are incredibly usefull

i want to ask you about how do i make intersting killing the powerfull samurai because the ninjas mision for them is to kill the samurai

You could go a traditional route and have them climb a tower, defeating his henchmen along the way with him on the roof of this tower. Depending on the player actions he could even escape the tower

Podrias seguir una ruta tradicional y hacer que suban a una torre, derrotando a secuaces en el camino con el en el techo de esta torre. Dependiendo de las acciones del jugador, incluso podria escapar de la torre

thanks for the answer

then what type of monsters will they fight or what type of challenges the heroes have to face

Depends on what sorts of monsters your samurai overlords have access to and willingness to use. Maybe they have an Oni on call? Maybe some sorts of guardian ancestor Specter or some other undead? Maybe mechanical Samurai Animated Armor? Depending on level, maybe they have to deal with some mini-boss samurai in the form of a Champion? Twig Blight Bonsai Trees?

thanks for the advice and i need to ask wich stats do you recomend me to use with the samurai?

Depends on the level range of your party. 

For lower levels: Bandit and Guard....maybe Orc or Scout or Bandit Captain

For mid levels: Githyanki Warrior, Knight and Veteran will work.

Just use the stats and re-flavor the monsters to look like samurai?

 thanks for the answer and my other question is if you see any failures in the story?

 and if i need to add some NPCs or something like that

It seems like a pretty political-intrigue laden story so you will likely need some NPCs to dole out clues and direct the players where you need them to be.