Complex sentence: Only Galvatron and Trion were left aware of what had happened to the Thirteen until Shockwave learned about them and sent his ores off to where he thought the twelve had gone.
Split into simpler sentences: Only Galvatron and Trion were left aware of what had happened to the Thirteen. Well, until Shockwave learned about them and sent his ores off to where he thought the twelve had gone.