Sentinel is a large machine that blocks the path upward in Mar-Uru. It hangs from the ceiling and has a white frame resembling the mandibles of a beetle. At the center of the frame is a ball-shaped protrusion with a single pink light, making it look as though Sentinel has a head with an eye.
Sentinel's appearance is not like that of the machines found in E-Kur-Mah, but more akin to the white armor worn by some of the Variants. This, and the fact that one must pass Sentinel in order to get to Athetos, implies that Sentinel is not one of the "Old Machines", but was built and installed by Athetos some time after his arrival on Sudra.
Trace encounters Sentinel while making his climb up Mar-Uru in order to reach Athetos. Sentinel appears to hang from the ceiling and will begin the battle with its frame open. During the battle, if Sentinel closes its frame, it will become invulnerable to all forms of attack. While the frame is open, Trace can damage Sentinel by attacking its eye in the center of its ball-shaped head.
When its frame is open, Sentinel will continuously fire purple laser bolts that travel slowly in circular patterns around itself. It will occasionally charge and fire a very powerful directed beam from its eye. If caught in the beam, Trace will suffer massive damage.
When its frame is closed, Sentinel will rapidly fire whitish missile-like energy from the upper portion of its body. These missiles will change direction completely at random, making it difficult to find a "safe" point in the room.
As the player has the Red Coat by the time they face Sentinel, a simple defensive strategy is to dash back and forth through the projectiles fired during both phases of the battle, firing at Sentinel's "eye" whenever a safe point appears. This becomes especially useful when coupled with the Inertial Pulse, which can bypass Sentinel's armor, allowing the player to fire from wherever they can reach its weak point.