Revan & Bastilla run off for adventures. Have kids. Revan runs off for another adventure to kill the Sith Emperor and is joined by the protagonist from KOTOR 2 (“The General”, they do actually give her a canon name but I forget what it is), and a Sith who was betrayed the Emperor and wants to kill him. Well surprise the Sith turns on our heroes, The General is killed and Revan is captured by The Emperor and imprisoned and the sith that betrayed them is rewarded by being made essentially immortal. Like he can die but he never ages. Anyway fast forward a bunch of years and SWTOR happens. The force ghost of “The General” calls out to the player character and guides them to Revan’s prison. Revan is freed and is like hey thanks, but my head is all fucked up, I need to peace out for a bit to get my mind right. Fast forward a bit and Revan has gone insane and restarted a forge to build an army of HK droids to kill anyone with even a small percentage of sith blood. The heroes are like Uhhh that’s genocide and Revan is like UNLIMITED POWAAAAAAHHH so you have to kill him. But his body blows up and you don’t really assume he is dead. Fast forward a couple expansions and a massive invasion is taking place simultaneously on Republic and Empire worlds and the two factions team up to combat the threat. Surprise, it’s Revan and a new army. You infiltrate his home planet and discover there is actually another Revan trapped there, a “light side” Revan who was split from the OG Revan (don’t ask) so you team up with “light Revan” and pretty much every significant NPC of the last several years to fight “dark side Revan”, you win, Revan is rejoined, and is like okay guys I’m gonna die now for realsies, thanks, and he goes poof.
The resulting devastation from Revan’s attacks led both the republic and empire vulnerable when ANOTHER empire from unknown space shows up out of nowhere and murks them both for several years. Turns out the emperor from this new empire is actually the old emperor the entire time, he just keeps reincarnating and fucking things up with an infinite plan B plan C plan D etc, honestly it gets comical. Anyway eventually you do defeat him and then the republic and empire to back to war with each other cause hey it’s an mmo and we gotta have faction warfare. Now you are caught up.