Red Eye (レッドアイ,Reddoai?) is a mini-boss from Sonic & Knuckles, as well as its locked-on form, Sonic the Hedgehog 3 & Knuckles. It is encountered in the first act of Death Egg Zone and is fought in two phases.
Red Eye itself occupies a metal pillar in the middle of the arena, with eight spheres spinning around it. These spheres spin around either the bottom of the pillar or the top, where the "red eye" of the boss is. When the spheres are spinning around the bottom, the player must make a difficult jump over the spheres, hit the eye and then back to the side again. When this is done, one of the spheres explodes, firing off spikes in eight different directions. Sometimes, when spinning around the bottom, the spheres will radiate out across the floor as well.
When all of the spheres are gone, the next phase begins.
The casing around the Red Eye explodes and drops away, enabling it to hover around the arena. Two spiked platforms emerge from the Red Eye itself and slowly rotate around it in either direction. The player must hit the red eye of the boss eight times. After hitting it once, the whole body of the mini-boss glows red and it will go berserk. At this point, the platforms will spin around very rapidly, making it very difficult to score hits. In addition, Red Eye will fire a large, sustained laser beam towards the floor. Eventually Red Eye calms down again.
An easy way to complete this phase is to jump on one of the platforms rotating around Red Eye and jump once directly underneath him. When the platforms rotate wildly, the player will not be thrown off, and once Red Eye calms down, the player has another chance to attack.
After this boss is defeated, the floor falls apart and drops away, but fortunately for Sonic and/or Tails, there is a platform underneath, which they fall onto before a High-Speed Elevator takes them up to the next act (although the signpost will fall off the screen if it landed far enough to either of the sides).
Red Eye's attack pattern in the first phase resembles the Flying Eggman boss.
If using the level select cheat to play this stage as Knuckles, then this boss cannot be defeated without seriously abusing the debug to place objects to stand on. This is because Knuckles' character cannot jump as high as Sonic can, and thus cannot reach Red Eye in its first form.
Like with Heat Arms, the battle with Red Eye is a sub-boss fight consisting of two phases.