If you're a fan of the previous Star Wars: The Old Republic expansion, Knights of the Fallen Empire, you'll be glad to hear that its followup is now live and available. The newly released Knights of the Eternal Throne brings with it new characters to fight alongside, a variety of (hopefully) interesting choices that will test your allegiance to the dark/light side of the force, as well as a brand new story campaign focusing on Vaylin - the most dangerously unhinged member of Valkorion's family.
Since that last part might as well be utter gibberish to anyone that hasn't played Star Wars: The Old Republic in recent times, here's a brief cinematic short showing off Vaylin's rise to power, and some of the story themes present in the Knights of the Eternal Empire expansion. Even if you don't really care about the lore I would still recommend giving it a look, its a surprisingly well told story!
If this has piqued your interested you can further explore the story by heading over to SW:TOR's website. Naturally, the big revelations are reserved for in game content, but on the positive side all of it is being given for 'free' to anyone with a subscriber status.
However, there is one portion of this whole storyline that's free for everyone to enjoy - the prequel for the Knights of the Fallen Empire. So if you're not sure whether this sort of storytelling is going to appeal to you, now you can at least test it out and see how it fares. Have fun, and do try to not destroy the universe!