Interesting plague that kills only those of noble blood instantly and only in a specific realm...
So it spreads real quick for a very short time or something?
Whats wrong with punishment from the gods? or black magic?
Its always fun to execute someone for it, put all the blame on that one person, that's what we humans do when tragedy strikes, offer a scapegoat.
Who knows, if you don't the problem will return?
Just crucify someone!