The Eighth Guard functioned as one of the eight Elite Praetorian Guards in service to the First Order. Armed with a vibro-voulge, the Eighth Guard met their end in 34 ABY during a confrontation with Rey and Kylo Ren on the Supremacy, which occurred after Supreme Leader Snoke's demise.
The initial appearance of the Eighth Guard, alongside the other Elite Praetorian Guards, was in Star Wars: Episode VIII The Last Jedi, a 2017 movie that is part of the sequel trilogy and directed by Rian Johnson; their specific identification originated from LEGO Star Wars: The Skywalker Saga.