The Eggrobo was constructed by Dr. Eggman in the period before the Deadly Six rebelled against him in Sonic Lost World and was specifically designed to be powered by the energy harvested from the Extractor. Before Eggman could deploy this mech, however, he was driven off from his operations by the Deadly Six when they broke free.
To get to the Eggrobo again and as well stop the Deadly Six from destroying the Earth, Eggman teamed up with Sonic and Tails. When Eggman and Sonic arrived at Lava Mountain, however, he faked his own death so he could get to the Extractor and the Eggrobo without anyone knowing. While Sonic took care of the Deadly Six, Eggman used the energy the Extractor had collected to super-charge the Eggrobo.
When Sonic and Tails got to the Extractor to shut it down, Eggman ambushed them in the Eggrobo. After having explained how he survived, Eggman revealed his plans to conquer whatever that was left of the world from the Extractor's effects with the same energies it had harvested. However, Sonic did not intend to let Eggman go through with his plan and engaged Eggman in the Eggrobo. In the end, the Eggrobo was no match for Sonic and was utterly destroyed.
Wii U/PC version
The beginning of the battle takes place in a narrow, straightforward platform aisle that Sonic runs on after being fired from a cannon. The Eggrobo will launch both of its hands at the player, and will shoot out a string of energy bullets in a straight line. After that, the Eggrobo will fire two lasers and move them across the pathway. Once this has been done, the Eggrobo will move both of its hands and then clap them together to crush Sonic in between. The player must double jump to dodge this attack. After taking substantial damage, the Eggrobo will move its hands while firing energy bullets and the laser attacks will sweep across the pathway much faster. After enough attacks, the player has the opportunity to hit the Eggrobo via Homing Attack on the Eggrobo's model parts until they reach to the Eggrobo's face. Once hit, the Eggrobo will fall on the ground and roll until it manages to pick itself up again. Sonic has a chance to use the Homing Attack at the Eggrobo while it is rolling to deal additional damage.
Nintendo 3DS version
The battle begins with Sonic already running after Eggman's mech. During the battle, Eggman uses powers of the Laser, Burst, Drill, Lightning, Asteroid and Quake. Between every attack, the power of the Eggrobo will be shown and will increase as the Eggrobo loses health. In Easy mode, it will start from 1000, then go up to 8000, then finally 10000. In Hard Mode, it will start from 1000, but temporarily go up to 5000 for one attack, then go down to 1000, then 8000, then finally, 10000. Finally as the boss of Lava Mountain Extra, it will start at 5000, then 8000, 10000, and will go past its limits as it reaches a power of 100000. When the Eggrobo is at its final power level, when Sonic has a chance to free two Wisps and one extra Wisp by landing an extra hit on it. Do this twice, and Sonic will have freed six Wisps, each Wisp granting Sonic some sort of strange power. After six Wisps have been freed, Sonic will charge at Eggman and use a super-charged Homing Attack powered by energy collected from the Eggrobo, forcing the player to rapidly press in order for Sonic to overwhelm the Eggrobo's defences and blow it up. Unlike the Wii U version, Sonic can transform into Super Sonic, but he will de-transform for the final attack.
- There is a glitch in the Wii U/PC version that allows the player to transform into Super Sonic. Ironically, the transformation is only allowed on the final attack, which is the opposite of the 3DS version. To do this, the player must have all Red Star Rings and obtain fifty Rings. As soon as the game becomes slow-motion for the final attack, hit the jump button for the Homing Attack then quickly activate Super Sonic.
- Since Super Sonic was never programmed for autorun sections, Super Sonic will not be running forward and the Homing Attack is the same as on foot, making harder to reach Eggman for the final attack. The path is circular thus making it easier to fall on the lava since the camera is stuck as the same position for the final attack.
- The Eggrobo has many similarities to various creations made by Dr. Eggman:
- The battle between the Eggrobo and Sonic has some similarities to the Nega-Wisp Armor fight from the Wii version of Sonic Colors.
- You have aim at the cockpit to deal actual damage to the final boss.
- When Eggman is defeated, he is ejected from his mech and sent soaring into the distance before exploding into an Eggman-shaped firework when he is out of sight.
- In order to do damage the player must chain multiple Homing Attacks together also similar to the Sand Scorpion from Sonic and the Secret Rings.
- After the player chains the Homing Attacks they will have another chance to deal extra damage.
- The boss shoots lasers at the player from it's two hands that get gradually faster as more damage is done.
- Much like the Time Eater from Sonic Generations, the Nintendo 3DS version uses more attacks than the console version. Also, like the Time Eater, both versions differ slightly in appearance (the cape on the Nintendo 3DS version appears much smaller).
- The Eggrobo is able to detach its arms, a similar ability which Time Eater owns as well.
- Eggman refers to it as his mech or his ultimate weapon, though both are merely titles and not the mech's name.
- The Eggrobo bears a great resemblance to the classic design of Dr. Eggman.
Wii U and PC version
Nintendo 3DS version
- ↑ Located in the final zone level's files, the mech's model is named "boss8_obj_eggrobo".