The Ruminator is a Fallen in Oninaki. It is a boss and is fought twice during Chapter 4 Part 2, first at Obbe Plains and then at Raom Meadows. It can also be fought on level 90 of the Sanctum of Rebirth.
Horrific Fallen clad in an armor-like carapace. They can use their arms like legs, and have a second head on the tip of their spear-shaped tail. Once they stop Defending, they launch their attack. Keep your distance, back up, or try to break their defenses; whichever works with your weapon.