THIS was a breakthrough for Indonesian horror? In that case they have a looooong way to go in terms of storytelling.
You've seen everything on display here. Without giving up any spoilers, suffice it to say, most of these scenes can be seen in "The Exorcist," "The Omen," "Zombie," and countless other horror films. In fact, "Satan's Slaves" is so derivative that it simply seems like an Indonesian version of countless other South Korean horror films.
The film had a slow build with a few jump scares, but the ending was a let down. Anyway, it is what it is. I suppose that coming from Indonesia, a religious country rife with censorship laws, "Satan's Slaves" would probably have been a breakthrough in that sense. Otherwise, meh. Nothing to see here, folks.