The Monarch was hiking taxes, executing townsfolk, and otherwise being really mean. Someone would have to find out what was going on with the <King/Queen> and then kill <him/her>.(Seriously, you will permanently kill the Monarch in this quest.)
Royal Assassination is a quest involving the assassination of the kingdom's monarch. It is a paltry aspect quest, available only when the kingdom has poor Well-Being, and costs 4 QP to undertake. There are two possible approaches to this quest.
Sly Slaying - solo quest for the Spy
The Monarch's Medicine - solo quest for the Physician
Note: Your Monarch will die. They are seen in the beginning of the quest in rags at the Pit of Judgement and die in front of you. You cannot bring them back. The Spy/Physician won't ascend to the throne, unless Brave New World is also completed. It is a good idea for your Monarch to have a child before this quest, the child will ascend to the Throne and items from the Monarch's inventory.