This is the walkthrough for End of the World as it is featured in Kingdom Hearts, as well as its Final Mix version. After sealing the Final Keyhole at Hollow Bastion Sora and his friends travel to the 'heart of the darkness', which is also where Ansem, Seeker of Darkness went, after being told by the Princesses of Heart.
Exit the area with the save point, Gate to the Dark, to enter the Final Dimension location, a vast space almost entirely devoid of matter. There are a total of ten treasure chests spread out here, located on tiny islands, and each of them are connected by a small invisible path (the tiny platform points in the right direction). Sometimes approaching or opening a chest will trigger a fight, the first one including new Invisible Heartless. Invisibles are among the most powerful non-boss Heartless out there, they come in groups, have a lot of health, and deal high damage. Aside from their melee attacks they can also turn into flame, forming a ring around you, which has to be avoided by jumping when it contracts.
Darkballs also appear, as well as an Arch Behemoth at the fourth chest, a recolored but somewhat weaker variant of the one fought at Hollow Bastion. Approaching the fifth chest triggers a fight against Angel Stars, flying Heartless that can use their wings to block frontal attacks. They resist magic, absorb Thunder, and can shoot long-range homing attacks. If they start to glow they will use one of three special moves, depending on the color, which can be disrupted by attacking them.
Before leaving you will have to fight another Arch Behemoth, after which you enter the Giant Crevasse. You will need to make your way to the bottom, although you can find the Meteor Strike for Donald by jumping into an area only accessible from above at the end of the canyon, followed by using the gap in the floor, which is revealed by defeating the enemies that spawn. Enter the blue whirlpool at the bottom to continue, which leads to the World Terminus.
The World Terminus is a series of separate areas with a teleporter to the next and to the previous area, with a large dark pillar in the center, provided you sealed the relevant world's Keyhole. Each pillar then takes you to a closed-off area from the related world, where you then fight related enemies, after which you can pick up a chest, and use the only exit to leave again. In the original version these chests contained copies of accessories dropped by bosses, but in Final Mix it was changed to synthesis materials for the most part.
You can find the Mighty Shield equipment for Goofy in the Neverland world, in the Galley, which becomes available after clearing the room normally locked by a yellow Trinity Mark. The second-last portal will be to 100 Acre Woods, which also contains a save point. Enter the final area, called the Laboratory, read the terminal, deal with the enemies that spawn, then use one of the portals to leave. Jump into the pit nearby for a bossfight.
Chernabog's bossfight takes place within the Evil Grounds, an area where your entire team has flight, akin to Neverland. Chernabog's weakpoint is his head, similar to Giant Ursula. He does not attack often, but can hit very hard, thus using Aeroga will be helpful in preventing fatal damage. He has a knockback move, blowing everyone away, which he can follow up with homing balls of light. His other main attacks are to breathe fire from side to side, and to raise one arm into the air to summon a pillar of fire.
Near the end of the fight he will wreathe his hands in flames and proceed to unleash a powerful melee combo on the area in front of him. His strongest move is used when he hunches over, then rears backwards to cause the volcano beneath him to erupt, engulfing the entire area around in him flames. It is best to immediately flee when he does this, as the damage will last several seconds. Note that you can't summon while flying, but you can land on the volcano's summit or his shoulder to quickly call Tinkerbell to help out. Lastly, equipping accessories that increase your fire resistance helps greatly.
After your victory you receive Superglide, the last shared ability, which is a much faster version of Glide, making getting around much easier. Enter the Crater, the now-inactive volcano, which leads to a series of rooms connected by portals. Finally you can enter the Linked Worlds area, where you have to face another Arch Behemoth, followed by waves of Darkballs, Invisibles, and Angel Stars. Defeat them all until the Heartless symbol is completely gone, allowing you access to the Final Rest area.
There is a save point here, as well as another door, which is the point of no return. Continuing means facing the final boss, although you can afterwards still reload your save and attempt the optional content. The main remaining objectives are the Hades Cup, plus two optional bosses in the matches that follow, and optional bosses at Agrabah, Neverland, and Hollow Bastion (in the Final Mix version). Lastly, there is also an incentive to grind synthesis materials for everyone's final superweapons, especially the Ultima Weapon Keyblade.
Before you leave, you can use the Linked Worlds area to repeatedly spawn Heartless, including the Neoshadows unique to Final Mix (their tactics and abilities change depending on how many are left), which can be useful for collecting synthesis materials for the Ultima Weapon. Final Mix also added two powerful new weapons to synthesize for Donald and Goofy. Enter the door once you are ready, then enter the secret cave area to start the bossfight.
Ansem bossfight 1
Ansem mostly leaves the fighting to his guardian; the Dark Figure. It will lash out with a series of ranged energy blades, which are automatically deflected by Aeroga. It will also frequently splay itself out in front of Ansem, blocking frontal attacks, forcing you to get around it, and it can also lash out with melee strikes. It's most dangerous ability is when Ansem shouts "Submit!", at which point the Dark Figure lashes out and latches onto whomever it hits. If you are affected your 'Attack' command will periodically turn into 'Freeze', resulting in the Dark Figure holding you in place. Meanwhile, Ansem will create an energy shield that blocks all damage and deals damage on touch, and attempt to hit you with it. He is slow on his own, but the Dark Figure can be a nuisance if you're careless.
If you refrain from attacking it at all it will deal minor damage to you randomly, but it is also the safest. Hitting Ansem on the other hand lowers the time it is attached to you. After his first defeat you have a moment to regroup and restore, after which you must follow him into the next section where you are cut off from Donald and Goofy, and forced to fight a Darkside alone. It should be very easy to defeat at this point, after which the second Ansem bossfight begins.
Ansem bossfight 2
Ansem will have less HP, but you are forced to fight hime alone. The Dark Figure no longer has the energy disc attack, but this time Ansem will use it to charge around the battlefield. Even so, Aeroga can still block it, and should likely be kept up permanently to mitigate his attacks. The "Submit!" attack remains as well, and will be far more annoying without your allies to take the hit. His newest and strongest attack is when the arena becomes shrouded in darkness, after which the Dark Figure enters the ground and flies out at high speed below your position. This will create a small shockwave, and due to the speed and upwards direction of the Dark Figure is best avoided by consecutive Dodge Rolls.
As Donald and Goofy will not be around to aid you it is recommended to rely on Elixirs if needed. Eventually Ansem will go down, after which the next bossfight begins.
World of Chaos bossfight
This time Ansem will have bonded with the World of Chaos battleship. He will be largely stuck in place, but due to his size and double-bladed weapon has a high attack range regardless. He will also continuously supported by laser beams, which are easy to avoid if you keep moving, but as you also need to attack him it can be easier to just tank the damage with Aeroga. Ansem himself will also deal out heavy damage with his quick combo strikes, so moving away whenever he prepares to attack also helps. Sometimes he also summons a few Bit Sniper enemies, which are generally a non-issue. World of Chaos can also blow you away with a large gust of wind, and drain your MP (which can be stopped by destroying the glowing light in its jaws). Halfway through the number of lasers will increase, but otherwise he gains no new moves.
After his defeat he will rest and recover inside the Dark Figure, after which Sora automatically enters the Portal to Darkness; a minor realm almost entirely absent of light. Destroy the Shadows then the snare-like object; the 'room core', to be ejected from the space. Back at the World of Chaos you must now destroy the artillery tentacles on top of its body, after which another portal opens. Destroy the mass of Darkballs inside, followed by the room core, to free Goofy.
Along with Goofy you must now destroy the Face, the big head on the bow of the ship. It will periodically lash out with thunder-based attacks, but otherwise is not much of a threat. You can take out the many artillery pieces supporting it if you want, but there is little real need. Afterwards enter the third portal inside its mouth and defeat the Invisibles inside, followed by the third room core, which frees Donald. Lastly you must destroy the main core in the center of the battleship, which allows you to fight Ansem again.
He will have the same attacks as before, which generally leads to Donald and Goofy being knocked out more often than not. He gains one single new attack; he will create a sphere of light and cast it out in front of him. This then expands, sucking in the three of you to its center where it deals continuous heavy damage, until it explodes. The only way to avoid it is by flying away at high speed, although it will saturate the surrounding area with tiny energy beams which also deal damage.
Either way, after dealing enough damage Ansem and World of Chaos will finally be defeated, which triggers the ending cutscenes and credits. Depending on the difficulty and your total accomplishments you will also unlock the bonus video; Another side, Another story.... Final Mix also offers the expanded version Another Side, Another Story [deep dive]. After the credits the game displays an ending screen with some combat-related statistics.