The Mortis Gods are from 3 episodes of the Clone Wars animated series and a few mentions during Rebels. However, you are correct, absolutely no set-up for introducing them. No context via the Ahsoka show whatsoever.
It ruined the payoff.....most folks have no idea who they are and why they are so important.
I think there was some ancient entities about the Force with like a Father and a Mother and Daughter and one was the Dark Side and one was the Light and the other was all about keeping the balance, and there was some hideous thing trying to mess it all up and driving random Jedi psycho so Luke and Ben Skywalker and Ben's Sith crush and her family had to fight it.
“Tolerance is the virtue of the man without convictions.” GK Chesteron
Inde muagdhe Aes Sedai misain ye!
Deus Vult!
*MySmiley*