Death Egg

The Death Egg is Eggman (Robotnik)'s main base of operations in the early Megadrive-era games.

Sonic the Hedgehog 2
The first appearance of the Death Egg is in Sonic the Hedgehog 2. As the final stage, it houses two boss fights: Mecha Sonic, and a giant Eggman robot. In the end, it explodes, and that is the end of the 1st Death Egg.

Sonic the Hedgehog 3
In Sonic the Hedgehog 3, the Death Egg, slowly floating above the clouds, begins a gradual decent towards Earth, eventually crashing into the floating Angel Island, forcing it into the ocean. In an attempt to repair the Death Egg, Eggman tricks local guardian of the Master Emerald, Knuckles, into collecting the Chaos Emeralds (currently in the possession of Sonic and Tails) for him, to use them to power the ship. Needless to say, this plan fails, as on the Death Egg's re-launch, Sonic sends it plummeting back to the island.

Sonic and Knuckles
The Death Egg is sent crashing into a volcano somewhere on Angel Island in the events of Sonic and Knuckles (or in the second half Sonic 3 and Knuckles for those playing it locked on). In a last ditch-effort to fix the Death Egg, Eggman hatches a scheme to steal the Master Emerald from Knuckles. He does so, and Knuckles finally sees Sonic and Tails as the good guys and Eggman as the bad guy. The three of them set off to catch Eggman, and Sonic and Tails leap onto the now-rising Death Egg from a tall pillar in the Sky Sanctuary. Sonic, with the power of the Chaos Emeralds, transforms into Super Sonic, has the final showdown with Eggman in outer space, wins, pulls the Master Emerald back to Earth, drops it safely on Angel Island, and peace and harmony are restored. As for the Death Egg... well, it exploded again. This time for good.

Sonic the Fighters
In the (questionably canonical) Sonic the Fighters the Death Egg II is built. Sonic and friends have a fight to the death to see who gets to fly up to it and destroy it. The player's character of choice flies up, has a (really frustrating) fight with Metal Sonic, followed by Eggman. After defeating Eggman, the Death Egg explodes, and that ends that.

Sonic Battle
Finally, in Sonic Battle, another Death Egg (most likely not the same one from the Megadrive games) makes a brief appearance at the end, although it is just a small arena for the game's final bosses, Doctor Eggman himself and a then berserk Emerl.

Sonic Drift 2
In Sonic Drift 2 for the Game Gear, the Death Egg is the final track on the hardest mode. Not really relevant story-wise, though.

Sonic Blast
Death Egg appears falling in the ending.

The Comics

 * Death Egg in the comics

Trivia

 * The Death Egg is a reference to the Death Star, the giant planet-destroying space station from Star Wars.