Quadraxis was probably designed and constructed by O-Lir, the last the Sentinel of the Sanctuary Temple. It has a similar design to Quads, sharing many abilities, but is several times larger and equipped with more advanced tactical systems.
In addition to an upgraded mater-antimatter weapon system, the Head Module of the Quadraxis unit is also capable of functioning without the main body, with limited capacity. The tactical control unit, within the main body of the Quadraxis unit, can send instructions when the modules are separated via high-sonic frequency transmissions.
Though Samus only encounters one complete Quadraxis unit, she does find several components of an incomplete unit in the Sanctuary Temple. The platforms she uses to navigate the room appear to be scaffolding used in the unit's construction. These pieces are identical to the components used in the Ing-controlled Quadraxis unit. When these components were made, and the reason why the unit was not completed, is unclear.
The battle takes place in a large circular arena with no safe zones; Samus is constantly taking damage from Dark Aether's atmosphere.
Shortly after Samus enters the Hive Temple, Quadraxis is possessed by an Ing and activates. Its legs will begin rotating around its body, and occasionally, one of its knees will become vulnerable. Even if Samus destroys the casing around a knee, it can still remain active. Each of the knees will become vulnerable when she uses her Boost Ball ability to damage the red orb at the bottom of each foot. Once several knees are destroyed, it will begin to use more powerful attacks, such as the Disruptor or its tornado attack.
Its primary weapon is the Annihilator Beam, which it will use to fire both normal and Disruptor blasts. Samus can evade the Disruptor blast with the Boost Ball or use the Screw Attack to deflect it. If she fails to dodge it, she will become temporarily stunned. Quadraxis is also armed with homing Missiles and a rapid-firing turret. Its legs are capable of creating shock waves.
After its main body is disabled, the Head Module will detach and continue to engage Samus. The main body will send transmissions to the Head Module, instructing it remotely. While receiving signals, the Head Module will raise an impregnable shield. If Samus uses her Echo Visor, she can target the antenna on the main body to temporarily disable it, causing the shield to drop. She can then target the receivers on the Head Module. Each time a receiver is destroyed, the Head Module will send out one Dark Quad.
After the three receivers are destroyed, the shield will drop and the Head Module will shatter the armor around the main body's legs, revealing a Spider Ball Track beneath each. The shield around the Head Module will also be permanently disengaged. Samus can stun the Head Module easily with the Light Beam or Super Missiles. After stunning it, Samus can climb the tracks and then boost to the magnetically charged surface on the head. After dropping a Morph Ball Bomb into each of the exposed vulnerabilities, the Head Module will crash into the ground and explode, leaving behind the Annihilator Beam.
- Quadraxis is the largest boss in the Metroid Prime series.
- In an interview with IGN, Bryan Walker of Retro Studios stated that the battle against Quadraxis was his favorite moment in Metroid Prime 2: Echoes.
- "Hear the words of O-Lir, last Sentinel of the Fortress Temple. May they serve you well. This used to be a great fortress, a safe haven for our people during the war. It also holds portals to the Ing Hive, the heart of their dark tribe. During the war, I built a number of mechanical sentinels to join me in battle. They were made to fight the Ing, and fight them they did...for a time. One by one, the Ing corrupted them. Now, they attack with no mercy. Many went to Dark Aether with their Ing masters." —O-Lir (Metroid Prime 2: Echoes)
- "The Quadraxis unit went rogue and entered the service of the Ing. Its primary weapon system fires destructive matter-antimatter blasts. It uses a powerful Missile battery for long-distance attacks. The Quadraxis is incredibly durable and well-armored, but its leg and foot joints are vulnerable. Damaging these joints will eventually render it immobile." —Metroid Prime 2: Echoes Logbook "Quadraxis" (Metroid Prime 2: Echoes)
- "Though the main body is disabled, the tactical control unit within it is still active. It will guide the Head Module in battle via high-sonic transmissions. Disabling this module will impair the Head Module's ability to fight." —Metroid Prime 2: Echoes Logbook "Damaged Quadraxis" (Metroid Prime 2: Echoes)
- "The Head Module of the Quadraxis is highly mobile and well armed. It is shielded by an impervious force field. Tactical commands are sent to the module from the crippled main body: sever this link to impair the Head Module's combat ability." —Metroid Prime 2: Echoes Logbook "Shielded Head Module" (Metroid Prime 2: Echoes)
- "Head Module is unable to receive tactical data. Force field offline. Unit is generating drones to defend itself while it attempts to restore tactical link to main body. Destroy sonic receptors on unit to prevent this." —Metroid Prime 2: Echoes Logbook "Stunned Head Module" (Metroid Prime 2: Echoes)
- "Module has lost its tactical link to the main body. Two points of structural weakness have been located on its hull: Morph Ball Bombs can cause damage to these areas. Scans indicate that the Spider Ball can attach to the Module's outer hull." —Metroid Prime 2: Echoes Logbook "Final Head Module" (Metroid Prime 2: Echoes)
- "This is a component of an incomplete mechanoid. Design similarities exist between incomplete unit and the rogue mechanoids of Luminoth design found throughout this area." —Scan Data (Metroid Prime 2: Echoes)
- "My favorite part about Metroid Prime 2 is the Quadraxis battle. It's the biggest boss that's ever been in a Metroid Prime game, just physically. It's also one of the most grueling. But I think it's just the most logically laid out of the boss encounters in that you have to take this massive structure down, bit by bit, in a realistic way," —Bryan Walker (A Space Bounty Hunter in Texas)