Perfect Chaos (Sonic Generations)

Station Square is the setting for a boss stage in Sonic Generations, originating from Sonic Adventure. Chaos, while in the form of Perfect Chaos, is fought in this stage as the Boss of the Dreamcast Era. Unlike the original battle, Sonic does not transform into Super Sonic to fight Chaos, as he does not possess all seven of the Chaos Emeralds.

Boss Battle
The battle with Perfect Chaos is mostly similar to the original battle in Sonic Adventure, however, without access to his super form, Sonic must use boost to avoid falling into the water and to inflict damage on Chaos.

The battle is, for the most part, a reprise from the final battle of Sonic Adventure: Sonic must travel through the saturated ruins of Station Square towards Perfect Chaos in both 2-D and 3-D perspectives whilst avoiding Perfect Chaos' attacks, which include energy missiles, giant whirlpools and water tentacles. The battle is split up into four phases. In the first and third phases, Sonic must boost into Perfect Chaos, moving up his watery body to attack his core. Unlike in Sonic Adventure, it does not matter what speed Sonic enters Perfect Chaos' body with. In the second phase, Sonic must traverse up the rubble of a building and use springs to attack Perfect Chaos' core with a Homing Attack. Perfect Chaos Revival starts like in Sonic Adventure for the fourth phase of the battle, but the battle style changes as Perfect Chaos causes several platforms to rise up from the watery area. Sonic must then jump across these platforms, whilst still avoiding Perfect Chaos' attacks, to hit Perfect Chaos' core with another Homing Attack, defeating Perfect Chaos.

The Hard Mode battle is much the same, but Chaos' attacks are more frequent, and another stage is added to the battle before the original fourth stage.

Trivia

 * Perfect Chaos is the only major boss in Sonic Generations that wasn't a creation of Dr. Eggman.
 * This is also the only boss in an era trailer that isn't a rival boss (Classic Metal Sonic was in the Classic Era trailer and Shadow the Hedgehog was in the Dreamcast Era trailer).
 * Perfect Chaos is also the only major boss from the past that doesn't appear in one of Eggman's structures.
 * The main difference with the two fights is that in Sonic Adventure you needed to be Super Sonic to travel across water, but since the release of Sonic Unleashed, Sonic has been able to do this himself so he didn't need to be Super in Sonic Generations.
 * A remix of the music from the original battle "Open Your Heart" by Circuit Freq plays during the stage, as well as a remix of the second phase Perfect Chaos Revival.
 * Perfect Chaos has his original roar which other monsters like the Biolizard also received with a pitch change. Chaos also now seems to have gained a new damage "cry", because when he gets damaged he will scream with a very sad tone in his voice. This new "cry" was actually in the original Sonic Adventure but was nearly silent compared to the music and explosion sounds effects of the battle, so it was incredibly easy to miss unless you listened very closely.
 * Perfect Chaos looks solid in Generations, while in the original version (Sonic Adventure) it was only water. In Mario and Sonic at the Olympic Winter Games, Perfect Chaos also looks solid, however, this is because he is frozen.
 * Perfect Chaos also appears to be more purple than blue, and possesses what looks like a green horn and green teeth. His skin looks like it is made of scales, which gives him an almost reptilian appearance, while that is not the case in Sonic Adventure.
 * However, it should be noted that Perfect Chaos had this reptilian look in the opening cinematic of Sonic Adventure, making it possible that he was always meant to look this way.
 * It has been explained that this was what Perfect Chaos was originally meant to look like (however, due to technological restraints was not possible at the time..
 * Oddly enough, it took 6 hits to defeat Perfect Chaos as Super Sonic, but it only takes 4 hits to defeat him as regular Sonic, which would seem to indicate that Sonic is stronger than he was when he last fought Perfect Chaos (when he required a Super Transformation to defeat him).
 * Also the two phases are different. The first phase in both games Perfect Chaos takes 3 hits to go to the next phase. The second phase Perfect Chaos Revival, Perfect Chaos takes 3 hits in Sonic Adventure but only one hit in Sonic Generations to defeat him.
 * Perfect Chaos is the only "monster" boss in the console version of the game, unless you count the Time Eater as a monster.
 * In the phases that launch Sonic into the air to home attack Perfect Chaos, if the player misses and falls, it's still possible to land a hit by boosting up his body, just like in the other phases.
 * This is the only boss in the game to have checkpoints.
 * This could possibly be because this is the only boss that is more of a stage than any other boss in the game.