I think the nature of it needs to explain why normal guards or soldiers aren't carrying this task out. 

It needs to be something difficult or unusual, or perhaps something secretive that the general public(and Dulcena's political rivals) shouldn't learn.  

Maybe we're acting, if not in direct opposition to, at least somewhat counter to the goals of the realm.  

Maybe we're going somewhere where the powers that be(whatever our governmental structure is) don't have any power, and so can't send official agents.