Face bald, belligerent Robotnik in his whirling Death Egg! — Instruction manual
Death Egg Zone (デスエッグ,Desueggu?, lit. "Death Egg") is the seventh and penultimate Zone of Sonic & Knuckles (and the thirteenth of Sonic the Hedgehog 3 & Knuckles). It takes place within the Death Egg and is also the last major Zone with two Acts. Act 1 takes place inside the ship, while Act 2 takes place outside the ship, with a view of the Earth from space.
The Death Egg was sent crashing into the top of the Lava Reef Zone after the events of Sonic the Hedgehog 3. In a last ditch-effort to fix the Death Egg, Robotnik hatches a scheme to steal the Master Emerald from Knuckles. As he carries out his plan, Knuckles sees Sonic and Tails as evil and the scientist as the good guy, but realizes that Sonic and Tails are the real good guys once Robotnik steals the Master Emerald right out from under Knuckles. The three of them set off to catch Robotnik, and Sonic and Tails leap onto the now-rising Death Egg from a tall pillar in the Sky Sanctuary Zone. Sonic battles his way through a horrendous Zone full of rocket launchers, antigravity chambers, electrical hazards, ring shooters, and some very mean Badniks. Instead of a single mini-boss as in the rest of the game, Sonic has to face two. First he faces Red Eye, then the Death Ball after Act 2, (controlled by the doctor himself, who is standing out of harm's way). After this, Robotnik flees, leading Sonic into the final fight against Great Eggman Robo.
Sonic then faces off against a giant mech named Great Eggman Robo. This giant robot looks like an EggRobo for the most part, except for its staggering size; its eye is as large as Sonic. The Robo attempts to squish Sonic with its three-fingered hands, of which each finger can be destroyed with three hits. After destroying the robot's hands, the mech stomps after Sonic, shooting flames from its nose. A quick hit on its nose tempts Robotnik to open the robot's large chest, where the Master Emerald can be seen. This huge compartment also contains a weapon powered by the emerald; a focusing lens which uses the Emerald's power to shoot a laser beam at Sonic. Although dangerous, this cannon is the only weak point on the entire robot. Sonic destroys it, and the gargantuan robot falls for good as the Death Egg begins falling apart due to its collapse. The scientist flees from the scene in his Eggmobile with the Master Emerald, but is quickly stopped by a few hits from Sonic. The Death Egg then completely crumbles, removing Sonic's foothold.
If Sonic obtains all the Chaos Emeralds, he will progress to The Doomsday Zone. Otherwise, the Master Emerald will remain in space and the game will end with the Floating Island remaining in the ocean but free from Robotnik's control.
The music played during the Great Eggman Robo boss fight is the exact same music played during the Big Arm boss fight.
Through the Sound Test/Act select, Knuckles can enter Death Egg Zone, but he is not able to complete Act 1 on account of his inability to jump high enough to attack the Red Eye. However, he is still able to complete Act 2, and should the player finish the Act as Knuckles, the game will send the player back to the Title Screen without playing a cutscene.
In the Sound Test/Act select, it is spelled without a space in the name, reading "DEATHEGG".