Hi people! If it won't be too cheeky I would like to add my 5 cents to this:
What about mixing all three ideas? I am thinking about a tyrannical king that has been consumed by a dark power that has consumed his mind, at the same time a plague has started to sweep through the lands (the source is unknown, but there could be a correlation between the fall of the once great and kind king and the plague). Add the princess idea: she could have an important role (power, ability etc.) that makes her very important for the resistance to gain the upper hand. 
Just an idea, hopefully you won't dislike my input too much ^^