Slay several Tidesylph ritualists around BBM coast. There are two ways to get in the cave: jump down from a hole on the island( 831, 23, -268 ) /waypoint 831.00, 23.00, -268.00, or from the underwater entrance( 739, 12, -259 ) /waypoint 739.01, 12.49, -258.92 (Shrillers and Foulers are placeholders).
Three Tidesylph ritualists spawn at a time on a platform above the water, but below the surface of the island. There's a hole that drops down to their location. They have about a 10 minute spawn timer.
Furthermore, if you drop down into the water there is an underwater passage leading to a larger chamber with four packs of ritualists mixed with foulers and pets. Underwater breathing is definitely recommended.