The two ended up playing a game of chess inside their heads. After climbing through a major rank, Achilles received a boon of increased memory capacity. It was now much easier for him to memorize and learn things.
Playing chess in his head was like a walk in the park to him.
It may sound impressive but his memory was nothing compared to the succubus.
According to the rumors about her, plus what he had learned from playing against her, Sia possessed an incredible photographic memory that would allow her to memorize almost anything she wanted. She also apparently had a thought-processing drive that was three times faster than a highly intellectual individual.
This made it incredibly hard to play a strategic game against Sia since she would be calculating dozens of moves and possibilities in advance.
If he was planning her five next moves, she would be visualizing his eight, or nine.