The Azure Dragon Manual. It was Damien's own creation, but its origin was a period long before he ever existed.
These techniques were first developed by Qinglong himself doing something similar to what Damien was doing now. He took the teachings of his Dragon Clan ancestors and transformed them into a technique geared specifically towards people who wanted to follow the same path as him, the path of water.
It started out simple, by having one feel water with one's body. Damien took August to an artificial river he'd created for this purpose.
It was actually an entire waterway that now ran along the Veridea Mountain and into the lands below, branching out and creating a completely new ecosystem.
It had to work on its own for August to truly feel "water." Damien made sure that his own influence was completely removed from the system before they ever arrived.