the Jester
Legend
Here's the thing - I don't want the Dwarven king to die from being stabbed to death - the circumstances of my story dictate that he is poisoned and dies. My example of the king who anticipates an assassination attempt foolishly doubles the guard but does not think to suspect his "loyal" servants. One of his servants laces his dinner with potent poison, and the king dies a painful death.
In this circumstance, stabbing the king to death A. is not reasonable if he is paranoid and keeps guards with him at all times and B. Does not conform to the story I'm trying to tell - it's a story about the Dwarven King trying in vain to avoid his fate by force of arms. Perhaps the assassin has motivations related to past sins of the king.
In my story, the servant is a lowly non-magical commoner. There is no wizard involved, and no magic. It's partially a tale of arrogance - nothing can harm me with my mighty armies, powerful spells, and heavy stone walls protecting me! There's a ridiculously simple solution - offer advantage to dwarves on their poison saves. After playing last night, advantage is a VERY nice thing to have but failure is still possible.
I immediately thought, "Why not slip rot grubs into his drink instead of poison?"
Solved, story more or less intact.
Nonetheless, I understand where you're coming from.