The Doomsday Zone

The Doomsday Zone is the final zone of Sonic & Knuckles and it's complete lock-on version Sonic the Hedgehog 3 & Knuckles. Under normal circumstances the zone is only playable by Sonic, but it is playable with Tails or Knuckles from the level select if Debug Mode is enabled. You must have all of the Chaos Emeralds to play the zone and can play as Hyper Sonic (Sonic 3 & Knuckles only) if you have all the Super Emeralds.

Boss
The Death Egg has once again crumbled from the loss of its main mech, but the Master Emerald is still in space. Now, as only Sonic and Dr. Robotnik remain, Sonic must summon the power of the Emeralds to chase Robotnik in a race against time. Sonic has fifty rings at the beginning and instantly transforms into Super Sonic or Hyper Sonic (the latter's case is if all the Super Emeralds are acquired). Since Sonic is in Super (or Hyper) form, his rings are steadily depleted, and he must keep picking up rings to help last him through the fight. He must also keep dodging asteroids which slow him down if hit, and missiles which render him unable to move or grab Rings for a second. Although only a few hits are unlikely to jeopardize the player's chance of success, even a slight lack of concentration can cause Sonic to sustain many hits and push him dangerously far behind. The A, B, or C buttons will make Sonic fly faster in any direction. If Sonic's rings are completely depleted, he loses his Super form, falls, and immediately dies. There is a section of the first part of this level that has Sonic dodging missiles in a slalom sort of fashion, and Sonic must stay between pairs of missiles as they move up and down. Rings usually occur in groups of three or six, buying Sonic that many seconds if all are collected, although there are patches of eight or twelve at the top and the bottom of the level. It is still possible to earn an extra life by having 100 Rings, but it is very difficult to achieve this. 80 is a good amount to have in hand when confronting the escaping mech. The ship is strong enough that Sonic can't directly damage it. It has a launcher on the bottom which fires homing rockets at Sonic. It also has a trio of cannons on its side which fires shots at Sonic. Neither of these will cause Sonic harm but will knock him back, hindering his efforts to destroy the ship. To defeat the ship, Sonic must make the homing missiles hit its head and avoid the ammunition it shoots. This must be done quickly as rings cannot be collected during this part of the fight. Once the ship has sustained eight hits of damage, it will fall and explode off-screen, initiating the final phase of the boss. A giant robot with a jet-pack emerges out of the blue outer shell, clearly carrying the Master Emerald. It resembles the Death Egg Robot in design, but with a more streamlined appearance and features similar to those of an EggRobo. It releases bombs if approached from above, and small spiny mines if approached from below. Sonic must catch up to it and touch it eight times to destroy it and recover the Master Emerald. This task should be done quickly, as Sonic may have trouble regaining rings faster than he loses them. What's interesting to note is that, within Hidden Palace Zone (three Zones previously), a mural in the background during the battle between Sonic (and/or Tails) and Knuckles depicts an abstract version of this battle. This mural would make an appearance in the Archie continuity as Sonic and Knuckles are lead through the Hidden Palace Zone.

Trivia

 * This level marks the first time in a recurring succession of Sonic games that the final level uses a Super form setting in a race against time and depleting Rings, only accessible through obtaining all the Emeralds.
 * The music in this zone is also used in Knuckles' version of the Sky Sanctuary Zone during the final battle against Super Mecha Sonic.
 * A remix of the music for this zone was originally supposed to be in the 3DS version of Sonic Generations for the boss fight against Big Arm, but it was later replaced by a remix of the final boss theme of Sonic the Hedgehog 3 & Knuckles.
 * In most versions of the game, the music will accelerate in tempo as Sonic runs dangerously short of rings, increasing tension in the player and urgency to keep collecting rings. This is similar to the Special Stages after some time has passed. This is missing in Sonic Jam.
 * The Doomsday Zone does not appear in either of the two comic book adaptations of Sonic & Knuckles, but Robotnik does pilot the mech in issue 53 of Sonic the Comic.
 * After Knuckles finishes this zone in debug mode, he is seen blue on the tornado and the original red one would be watching him at the end. He is also seen with two Master Emeralds, one in his hand and one on the back of the tornado.
 * The final part of the boss appears in Sonic and the Black Knight in winning fanart.