Roy became aware of the existence of Chaos with the help of his system. The notifications it issued to him when he killed the Red Mist Walkers also helped him conclude that it has corrupted the spirit of the Dragon's Pond.
But, of course, he couldn't outright tell them that.
The existence of the system was a secret. A secret is best kept when it is known by only one person. Roy had no intention of ever revealing it to others, at least not in this lifetime. He was quite certain about that.
"I have a skill that allows me to determine whether someone is a chaotic being or not when I killed them. It activated when I took down the two Walkers, signifying that they accepted Chaos."
Roy mixed lies with truth, giving them a valid answer before he pointed at Tevenlor and continued,
Phew~! Last Four to Eight chapters are left before the first part of this arc ends!