Complex sentence: As the acting leader, Newt calls a "Gathering" to decide the fate of Thomas for going into the Maze without permission, and it is decided that Thomas be made a Runner after serving one day in "the Slammer" (jail).
Split into simpler sentences: As the acting leader, Newt calls a "Gathering" to decide Thomas's fate for entering the Maze without permission. It is decided that Thomas be made a Runner after serving one day in "the Slammer" (jail).