The book series was introduced near the end of Legends' run and focused on Luke Skywalker's journey to take down the Force entity Abeloth, who began rampaging through the galaxy in her attempts to bring chaos to the cosmos and become a true God of the Force. So, it says a lot about the Force entity Abeloth that the Sith were willing to team up with the Jedi in order to bring her down in Star Wars: Fate of the Jedi. It's no exaggeration to say that their enmity is responsible for practically every major conflict in the galaxy's history. In practically every story, be it canon or Legends, the Jedi and Sith always come into conflict, many times with catastrophic results for all involved.
In Star Wars, there is one single constant: the Sith hate the Jedi and vice versa.
