Envoy of the Depths is a boss you can fight in Red Hood’s Woods, he’s located in the Shore of Depths area that you can reach through the Lake Woods. He will be squirming in the water and suddenly jump out towards you as you get closer, this will initiate a fight.
By killing him you will learn the Silence Bubble skill, and will also be able to go further in the area by jumping into the lake, reaching the Depths.