The oculus was a device created by the Attendants and used by Supreme Leader Snoke of the First Order, and the Sith Eternal. Stationed in Snoke's throne room on board his Mega-class Star Dreadnought the Supremacy, it functioned as a sophisticated magnifying glass, enabling its user to better see things that were kilometers away. In 34 ABY, during the Battle of Oetchi, when Rey was brought before Snoke, he forced her to use it to watch the destruction of Resistance transports fleeing the Resistance warship, the Raddus, for the planet Crait.
There was also an oculus device in the Sith Citadel on Exegol.
- Star Wars: The Last Jedi: The Visual Dictionary