Two days after Gravis talked with the twins, something surprising happened. The twins had left the Greys. Elder Byron had informed Gravis that they had changed their mind and didn't want to be part of the Greys anymore. Gravis was a little confused but also felt like he understood the reason.
After they had talked, they had probably decided on their future path. Maybe they would return to the Water Sect? Maybe they decided to stop increasing their power? Gravis was not sure, though he thought the possibility that they were no longer interested in cultivation was more likely. There was basically no better group to temper their wills than the Greys. Leaving the Greys probably meant that they finalized their decision to no longer temper themselves.
