I'm writing a fiction right now in a fantasy setting, and at one point in the story a country falls into anarchy after the royal family is wiped out. Has such an event actually happened before? I'm trying to figure out realistically what would occur in that situation. If a new monarch were to be chosen, how would it be done, or what would be the most likely outcome of the country's government?
That has indeed happened before, several times actually. Maybe you might not be aware, but kingship for example in the Holy Roman Empire wasn't for one royal family only. In the HRE there were several families, the most famous were the Karolinger (Carl the Great), Ottons and - in the later days - Habsburger. After the death of a monarch, the nobility chose a new king. In the later days (under Carl IV. Luxemburger IIRC) he declared 7 authoritys to elect the king - the bishops of Mainz, Trier and Cologna, King of Bohemia, Duke of the Rhein, Duke of Brandenburg and Duke of Saxon who elected the king. The election was full of politics, and mostly one of this council (the king of Bohemia, who was in "his" country too elected by the nobility of bohemia) was elected, and if not, then it was someone who wasn't too powerful (e.g. the first Habsburger-King Rudolf), so these 7 "Kurfürsten" were actually the most powerful in the empire.