The Praetorian Guard

The Praetorian Guard was a special unit which existed (ostensibly) to protect the Roman Emperor, but they also assassinated emperors and crowned new ones, for the right price. They were originally formed by Octavius Augustus, the first emperor, in 27 BC, and were disbanded by Constantine the Great in 312 AD.

Sources:

“A Ditionary of The Roman Empire” by Matthew Bunson

https://www.history.com/news/8-things-you-may-not-know-about-the-praetorian-guard
https://www.britannica.com/topic/Praetorian-Guard
https://www.britannica.com/biography/Augustus-Roman-emperor
https://www.worldhistory.org/Praetorian_Guard/