In a monarchy, who has the legitimate power to rule

The monarch, being the king is the supreme authority. It can also be a queen as it has been with queen Victoria in Britain

Ongoing civil wars western roman empire finally collapsed476 c.e. romulus, the last of theroman  emperors in  the west, was overthrown by the germanic leader odoacer, who became the first barbarian to rule in rome. the order that the  roman empire  had brought towestern  europe for 1000 years was no more.