Sylas already knew what happened the moment that fool of a priest accepted the golems back.
In truth, Sylas didn't know or understand how the golems worked initially. But between the time that he summoned the Great Priest and the end of the battle, he had had almost half a day.
In that time, he had analyzed the golems, only to find that they might have been complicated mechanisms… but at the end of the day, they were built off the same fundamental laws that he was used to.
Runes.
The moment he understood that, he was able to deduce the locations through which the golems were controlled and implant his own Will into them.
To be clear, he couldn't control the golems. That would take significantly more study and time he didn't have. Although controlling golems wasn't as difficult as knowing how to draw every Rune in its body like he would have to for the scorpion army, it was only two or so steps beneath that.
