Master Core (マスターコア Master Core) is the secret final boss of Super Smash Bros. 4. It appears at the end of Classic Mode, fought after weakening Master Hand and Crazy Hand on higher difficulties. It appears as a mass of black particles known as the "Swarm" that take on various forms depending on the difficulty level and battle progression.
Master Core can only be battled if the Intensity is set to 5.1 or higher. On the 3DS version, Master Core only appears if the player chooses to take the black path to fight both Master Hand and Crazy Hand at the end of Classic Mode; at Intensity 8.0 and above, there is no option except the black path. On the Wii U version, players will always encounter Master Core as long as they meet the Intensity requirement.
The battle between Master and Crazy Hand begins as usual; however, once the Hands lose approximately 100 HP (150 HP in co-op mode), Crazy Hand disintegrates into dark flames while Master Hand flails about and rips open to unleash the Master Core entity. The music changes to Master Core's theme and the background changes to a swirly multi-colored warp (or a solar flare-like background in the 3DS version). Master Core has HP like other bosses, but it is never shown to the player; in the 3DS version, Master Core's character portrait on the bottom screen is concealed by a cloud of Swarm particles, while in the Wii U version, the HP meter is simply not displayed to the player at all. It is somewhat possible to tell how much remaining health each form has, as every 50 HP of damage dealt to each of the first three forms will cause a hole to burst open in its body, while Master Shadow will gradually shrink as it is damaged.
Master Core begins the fight in a certain form based on the difficulty setting. As it takes damage, black Swarm particles will spew out from the area it is last hit. Once a form is defeated, it progresses to the next one. As the difficulty setting increases, the forms will attack faster.
Master Giant appears as Master Core's starting form at Intensity 7.5 and higher on the 3DS version, and Intensity 7.0 and higher on the Wii U version. It appears to be a large humanoid creature.
Master Giant has the following attacks:
After defeating the Master Giant form, Master Core will transform into Master Beast.
The Master Beast form appears first on Intensity 6.0 to 7.4 in the 3DS version and Intensity 6.0 to 6.9 in the Wii U version, or when the Master Giant form is defeated. It appears to be a large quadrupedal scorpion-like creature.
Master Beast has the following attacks:
After defeating the Master Beast form, Master Core will transform into Master Edges.
This form can be encountered first on Intensity 5.1 to 5.9, or when the Master Beast form is defeated. The swirling background on the Wii U version turns yellow at this point. It appears as a group of five swords, one larger than the others. Any of the swords can be damaged equivalently. In PAL regions, this form is known as Master Sabres.
Master Edges has the following attacks:
After defeating the Master Edges form, Master Core will transform into Master Shadow.
In this form, Master Core appears as a copy of the player's character, including any equipment, custom movesets, or alternate costume the player's character might have. If the player is using an alternate form of a character, such as one of the Koopalings or the male Wii Fit Trainer, these will be copied as well. At the start of the fight, Master Shadow is noticeably larger than the player's character, but will gradually shrink as its health drops, as well as the black becoming more transparent, somewhat revealing the true colors underneath. Unlike the previous forms, there is no difficulty level where this is the first form encountered, and it can be defeated by KOing it like any normal character as well as by depleting its HP like the other Master Core forms. In the Wii U version, if Classic Mode is played on Co-Op mode, Master Shadow will only copy the first player and has a blue outline.
Master Shadow's AI varies, but it normally is passive enough to allow for multiple blows on it; there are times however that it can be quite unpredictable and may instantly land killing blows as a punish, especially if the player has sustained high damage from the prior forms and if they make a reckless move.
After defeating the Master Shadow form, the music will abruptly cut out. Master Core will then either transform a final time into Master Fortress (only on the Wii U version, at Intensity 8.0 to 9.0, and only on single-player mode) or finally reveal itself (on the 3DS version, or on the Wii U version if the prior conditions are not met).
The Master Fortress form is exclusive to the Wii U version of the game, and only appears on Intensity 8.0 to 9.0 on single-player mode after Master Shadow's defeat. When this form appears, a different background music will play, the time limit will be automatically extended by three minutes, and a special Heart Container will appear for the player to use, which resets the player's damage to 0% like in All-Star Mode. If the Heart Container is not used, it remains on the stage and can be picked up at any time by exiting the Fortress.
Master Fortress is effectively an entire stage resembling a human body that must be traversed, and the player must search for and destroy a total of four weak points to proceed. These weak points resemble hearts, and beat audibly when the player is close. When Master Fortress first appears, a barrier will appear and deal set knockback (albeit no damage) to prevent the player from entering the fortress before it stops forming. Once inside the fortress, shadowy versions of enemies such as Stalfos, Geemers, Plasma Wisps, and Lethiniums will appear and attack the player.
Master Fortress appears in two waves, with the first wave containing the first two weak points and the second wave containing the last two. Only one weak point is accessible at the start of each wave; destroying it opens a path through the Fortress to the wave's second weak point. The second weak point in each wave has the ability to defend itself by summoning several enemies nearby the player when attacked. At the end of each wave, the background music will cut out.
The first weak point is reachable by a straightforward path with few enemies and no traps. All future weak points feature the presence of the Fortress's numerous Danger Zones, which take the form of either subtle yellow outlines on the walls or large pools of acid. If the player touches a Danger Zone when their damage is 100% or greater, they will be instantly KO'd; if their damage is less than 100%, they will instead take 25% damage with considerable knockback. At the same time, the enemies spawned in the Fortress can be knocked into the Danger Zones to defeat them more easily. Thanks to the Fortress's walls, these Danger Zones are effectively the only way to be KO'd while inside the Fortress (thus there is effectively no way to be KO'd while going for the first weak point, and no reason to pick up the special Heart Container until after then).
Upon destroying the second weak point, defeating the first wave, the screen will fade and the player will be ejected from the Fortress (giving them an opportunity to use the Heart Container if they have left it behind earlier).
Entering the Fortress again will face the player with the second wave; the background music will also change once more. The second wave modifies the stage layout to include new paths as well as more enemies and Danger Zones. As before, the third weak point must be destroyed before a path opens to the final weak point. This final weak point is located in a cramped area virtually surrounded by Danger Zones, as well as containing moving platforms which can disorient the player. The limited space enables the enemies summoned by the final weak point to quickly knock the player into the multiple nearby Danger Zones to inflict very heavy damage. Destroying the fourth and final weak point will defeat Master Fortress.
After defeating the Master Fortress form, Master Core will finally reveal itself.
After all previous forms have been defeated, the black Swarm particles will completely cover the screen briefly, before abruptly dispersing to reveal the Core itself in the center of the stage. At this point, the background music goes silent and the timer will stop, and on the 3DS version, the black particles covering Master Core's HP on the bottom screen will also disperse, revealing a new character portrait for the boss and its damage percentage starting at 0%. On the Wii U version, the swirling background will turn blue.
The player now has approximately 45 seconds to defeat the Core by knocking it off the stage. The Core appears as an object similar to a Smash Ball and will go flying upon being hit, depending solely on how much damage it has taken (which means attacks that usually have a fixed knockback will work as regular attacks against it). If the Core does not get sent beyond the blast line with each hit, it will fly back to the center (or teleport back if it gets stuck under the ledge). During this time, the Core has no attacks, and will gradually fade during the 45 seconds from a bright prismatic color to a deep red.
If the Core has not been defeated after 45 seconds, it will begin to spin rapidly and rise to the center of the screen, then charge up and unleash five red waves. Being hit by any wave will inflict 50% damage and one-hit KO the player (similar to Tabuu's Off Waves). The core can still be attacked while floating during this time; if it is, it will stop releasing waves and give the player some time to KO it. However, if it isn't attacked for a short period of time, it will unleash them again. Like the Off Waves, these waves can be avoided with precisely timed rolling or sidestep dodges. After all five waves (if the player still has lives left), the Core will self-destruct, causing the player to automatically win.
In both versions of Smash 4, the track "Master Core" plays directly after Master Hand's transformation, and it can also potentially play on Final Destination in the Wii U version. Two extra tracks, "Master Fortress: First Wave" and "Master Fortress: Second Wave", also play during the first half and second half, respectively, of the Master Fortress transformation. As the Master Fortress transformation does not appear in the 3DS version, these two tracks are only available in the Wii U version, and they can also play on Final Destination when they are unlocked.
Like most original tracks in Smash 4, "Master Core" features several melodies and reprises from Smash 4's official theme. Of note is that periodically in the track, the music cuts out and is instead replaced by a series of beeps that are in Morse code. When decoded, the beeps spell out the phrase "MASTER CORE".
Of note is that Master Fortress's fourth chamber interacts with the "Master Core" track. When players approach the fourth and final weak point, a faint heartbeat can be heard, beating in sync with the track.