Darth Sidious And The Failure of The Jedi
The Force Awakens deliberately sidestepped the controversial prequel movies in order to focus on the nostalgia of the original trilogy. The Last Jedi, meanwhile, isn’t afraid to reference George Lucas’ oft-hated films as it calls back to the tragic events of Revenge of the Sith.
When he’s trying to explain to Rey that the Jedi’s legacy is nothing but failure, Luke references the fact that, even when the Jedi were in their prime, they were foolish enough to let “Darth Sidious” rise to power and corrupt the Galactic Senate into the Empire.
This, of course, is the main thrust of the prequels, with Chancellor Palpatine finally revealing himself to be a Sith Lord in the third installment. Luke also refers to the Emperor by his Sith name for the first time in the series, as it was introduced in the prequels.