In the games "Knights of the Old Republic" 1 & 2, it was established that Darth Revan turned to the Dark Side and attempted to take over the Republic in response to his discovery of some distant evil. His attempts to conquer the Old Republic was born from a desire to present a united galaxy against this unknown evil. During the events of the first game, we learn that
Revan's original plan was interrupted when the Jedi captured & mind-wiped him, thus turning him into the protagonist of the game.
Afterwards, Revan leaves the known galaxy in order to prepare for this inevitable confrontation, but is not heard from again despite several mentions in KotOR 2: Sith Lords.
In the MMORPG "The Old Republic", the recent expansion "Shadow of Revan" showcases
the return of Revan, now a being split into two halves (Light & Dark). It also reveals that the "great evil" he sought to protect the galaxy from is the Force essence of an ancient Sith Emperor.
The latter felt rather shoe-horned in, and many fans feel that it doesn't fit with the clues from the first and second KotOR games. Was this the plan all along, or did the nature of the threat change in-between KotOR and TOR?
As always with questions of this nature, an answer like "We don't know, they never explained that." is perfectly acceptable.