Metallic Madness boss

For the unreleased final round, see Final Fever (R9).

Psyco-Egg (as named in concept art) is the seventh and final boss encountered in Sonic CD, in Metallic Madness.

Final Battle
Once Sonic the Hedgehog enters the final arena, Doctor Robotnik will appear in his final machine, a mechanism which contains four, rotating multipurpose blades. It will hover above the ground, spinning the blades at a quick rate, and will eventually drop down trying to crush Sonic by rotating his craft to use the blades as "feet". Eventually, he'll allow the blades to detract and use them as weapons, but this is the first advantage to defeating the boss. Once the scientist is hit, a blade will fall off, but the other blades will spin quicker than the previous time.

Like the previous phase, Robotnik will attempt to crush Sonic using the blades, and will hover back in the air to try and use them as projectiles. Another hit and the second blade is down. Upon the third phase, the doctor will try to shoot the blades from above the screen in a diagonal fashion. One hit more and it leaves the fourth blade, which spins at a super-quick speed. In a desperate, final attempt, Robotnik will use the blade as a spring to stomp on Sonic, but this proves to be a mistake, as there is no other exterior protection outside of the machine, and the last hit causes it to explode.

Ending
With that, the whole area of Metallic Madness will begin to detonate, and Amy Rose appears on the scene. Sonic will escape along with Amy outside of Robotnik's exploding fortress, and this triggers the rest of the ending.

Depending on whether or not the Time Stones are collected, the ending varies. In the bad ending, Little Planet isn't completely freed, and Robotnik will escape with a seventh Time Stone. However, Sonic will hurl a rock towards Robotnik escaping in the air, causing his Eggmobile to explode, and the whereabouts of the remaining Time Stones are unknown. After the credits, time will reset and Little Planet will remain chained to the mountain. In the good ending, Little Planet is freed from Robotnik's control, and it will sow its seeds on Sonic's world.

Unused Extra Boss
On May 16th, 2012, Christian Whitehead revealed concept art of a additional boss round called Final Fever exclusively for his remake of Sonic CD. It was intended either when the player was about to unlock the good ending or had all the Time Stones. Whitehead had this to say about it: ''[....] it’s colourful since it was intended to be an all Time Stones boss (good future only). Time flows freely, and Eggman’s machine is essentially invincible. The idea being Sonic can reach warp speed at any opportunity (instead of getting the warp posts) and must do so to “wind back the clock” to damage the boss. Some parts are running, some areas the screen is locked. I was slightly worried the idea of running to warp might not be self evident (no warp screen, the level and boss would change in real time with some neato effects).''

For unknown reasons, the idea was scrapped, but he said that it could possibly be released as DLC.

Trivia

 * It is likely that the boss was intended to be named the Psycho-Egg.
 * One piece of concept artwork for the Psyco-Egg revealed that machine was going to be more of a robotic exo-suit at one point. The boss in the final game merges the design of that version and another design of the R8 Boss.
 * The music track is named Final Fever, and each soundtrack has a very different variation. The JP soundtrack uses a completely different theme exclusive for this boss that was later remixed in Christmas NiGHTS into Dreams... for Sega Saturn. The US version uses a slightly remixed, extended version of the regular boss theme instead.
 * In the original game, the spinning blades used a rather choppy animation, which was smoothed out in the 2011 remake in a similar fashion to the Special Stages.