Please Pay the Barbarians
In its twilight years, the Western Roman Empire was a shadow of its former self. The empire was stricken with corruption and strife, while several people began making moves to claim the throne for themselves. Orestes was one of those people.
Now, Orestes was a general, so you would be forgiven for thinking he had an army to help him become emperor. Funnily enough, he didn't. He solved this problem by hiring some German barbarians to do his bidding. These Germans were led by their king, Odoacer.
Barbarians don't come cheap, though, so Orestes and Odoacer struck a deal: if they helped Orestes become emperor, he would grant them lands within Italy. They agreed to this, and went on to conquer the country.
Orestes and Odoacer went on a two man power trip, and they successfully claimed the throne. Orestes put his son, Romulus Augustus, on the throne, and he was happy. Odoacer was excited to have some new lands in a brand new neighborhood, and he was happy.
Unfortunately for Odoacer, Orestes showed his true colors as a jerk and a doofus. He reneged on his promise to grant land to the Germans, and they were apoplectic.
The Germans revolted, and what a revolt it was. They captured the city of Pavia before marching to Rome. Orestes was killed, Romulus Augustus abdicated and was exiled, and Odoacer declared himself the King of Italy, effectively ending the Western Roman Empire in such a pitiful and anticlimactic fashion.