I got to thinking last night about how the people in the universe on the other side of the Fault have red eyes and are basically evil folk.
Then I remembered that when SuperNova appeared to Malik Tarcel, he had red eyes. And we already know that he's evil. :)
Saal said that he heard Malik's torture and came to rescue him. Even though this was before the Fault was ripped open, it's possible that SuperNova could have felt his pain through the Nova Force.
In the past, we've seen that a Nova (under the right conditions) can open a portal into another universe. So it seems very possible that Other Universe Saal came through.
And the fact that he would possess all the Nova Force of his universe makes him one bad dude.