Master Core (マスターコア Masutā Koa?) is a boss in Super Smash Bros. for Nintendo 3DS and Wii U. It is the final boss of the game's Classic Mode and Master Hand's true form when it is played on 5.1 intensity or higher. In addition, the black path must be taken during the fight; taking the blue path will only have a fight with Master Hand. If playing on intensity 8.0 or higher, the black path will be the only one available, meaning that a fight with Master Core is mandatory.
When Master and Crazy Hand appear on the field, they will have set amount of health (which varies on intensity). The player only needs to take out 90 HP from one or both Hands. When this is done, Crazy Hand disintegrates while Master Hand's glove rips open, revealing the Swarm.
If playing on an intensity 5.1 or above, it will only use its "Master Edge/Sabre" form and "Master Shadow" form; starting at level 6.0, it will introduce its "Master Beast" form; and at level 7.5, its "Master Giant" form will appear. In the Wii U version, on intensity 8.0+ and defeating "Master Shadow", it will introduce another form; "Master Fortress".
Master Core usually uses the formations in the following order:
- Master Giant (intensity 7.5+) > Master Beast (intensity 6.0+) > Master Edges/Sabres > Master Shadow > Master Fortress (Wii U only) > Core
To proceed to the next form, you need to deal enough damage to the form to force changing it.
The Swarm is the first form Master Core ever uses. It uses this to transform into any form it has (see above in the orders of the form). This form cannot be hurt.
The Master Giant form is a large humanoid creature. It appears as Master Core's starting form at Intensity 7.5 and up. The Master Giant has two physical, energy and one mixed attack. The first melee attack is a seemingly harmless swipe horizontally across the arena, before it the area it hit flashes and deals damage to anyone caught in it. This attack cannot be blocked by a shield. The second attack has a similar effect to Master Hand's grabbing attack, however, two of its four arms will open up, while the head will rip open to reveal a rift. If a character gets caught by one of the open arms, it will drag them into the rift, after which it will close and deal rapid but low damage before ejecting them out onto the platform for a larger amount of damage.
The first energy attack the Giant uses creates six electric-based orbs at the top of the level which move horizontally, after which it lifts the platform up to the top to hit the player with them. The second energy attack the Giant uses creates a large tumor-like object in the head, after which it will drop it, shattering into four orbs which will later explode in a X Bomb-like fashion. If the tumor-like object hits the player, it will bury them, and the orbs' explosion does very high damage.
The mixed attack has the Giant's head open to reveal a thin X-like rift which sucks players in. In addition, four energy orbs fly in from the left of the platform. If the player gets caught in the rift, they will be dealt with rapid damage similar to its second melee attack. It also has a similar emitting attack to a ReDead which can KO players by sending them flying towards a blast zone. However, it rarely uses this attack. This deadly attack can be avoided by dodging the attack just before it screeches.
The Master Giant hangs behind the platform rather than using it, similar to summoning Andross from an assist trophy. Master Giant's health is depleted by attacking its head, compared to the other forms where hitting them anywhere does damage.
Master Beast is a giant beast that takes up a good amount of the platform. It appears as Master Core's starting form at Intensity 6.0-7.4. It has three physical and one energy based attack. One of its attacks involve jumping into the foreground before biting at the player's position, similar to Master Hand's Jet attack. Another attack has the Beast protrude spikes on its back, after which they will appear at the player's position on the ground, similar to Dark Samus' spike ability and can be avoided by being high enough in the air or a well-timed dodge. It can also jump around the platform, dealing damage if it hits the player at the beginning or end, and knocks the platform around violently. The energy attack is rarely used, but a bolt of lightning will strike the Beast, dealing damage to anyone near it.
Master Edges/Sabres are sword forms. It appears as Master Core's starting form at Intensity 5.1-5.9. They begin the fight by one large sword, which then appears as the large sword and four smaller swords, which then move to the eastern edge of the platform. It will not attack back while it "shows off" its swords. It has two energy and two physical attacks. The first energy attack launches four small orbs in two sets, before homing in on the player. They disappear upon hitting them. The other energy attack launches two large orbs in two sets. Both attacks have a sling-shot effect; they appear from the edges, move slightly towards the left before flying back at them/the player. The second energy attack can be avoided if you fly high enough, as they do not home in on the player.
They also have two physical attacks. The first physical attack has the Edges getting ready to slice at the player. If the Edges manage to reach the player, they quickly slice and dice them, before the large sword lands a powerful slash. The second attack has the smaller Edges line up in a very thin X formation, while the main Edge is hovering over the player, similar to a Spiny Shell. The smaller Edges will begin attacking, one at a time. After the smaller Edges finish attacking, the main Edge will launch a powerful attack at the player, which causes massive damage & knockback if it hits. This can be avoided by dodging the small Edges, then using a perfect shield against the larger Edge.
In the penultimate phase, Master Core forms a bigger copy of the first player's character (ex. Player 1 is using Kirby, it will make a larger Kirby), and if the used character has been customized, the Shadow will also copy all equipment and abilities that they have. The Shadow will also use shields during the fight. As the Master Shadow becomes weaker, the clone will also become smaller. To defeat the Shadow, you can either chip away at its health or KO it out of the arena. A powerful character with strong smash attacks should be able to defeat it easily in two hits.
The Master Fortress only appears in the Wii U version of the game. Once all the remaining forms are defeated (excluding the Core), the platform will land on the Master Fortress. The player is also given a Heart Container and 3 extra minutes to complete this form. In addition, various enemies in shadow manifestations from Smash Run appear and attack the player. It is highly advised to deal with projectile launching enemies quickly as their damage can quickly stack up compared to the melee using enemies. To proceed towards the next area, attack the heart-like core hanging on the fortress to unlock the next area.
The first phase of the fortress has two cores to get through. The first core is protected by three enemies, with one additional one showing up at the outside. Once said core is destroyed, it reveals another area, which has more enemies. The second core is protected by more enemies and has acid walls/pits which serve as Danger Zones for both player and enemy and can deal very high damage if the player touches them. Once this core is destroyed, the player is sent back to the platform and the second phase begins. At this point, the player must reach the third core, which takes a path similar to the second core but lies at the other end of the fortress.
When the third core is destroyed, the swarm moves around to create the final fortress. This fortress is significantly longer compared to the others, having significantly more acid pits and walls. More enemies will also appear, although they are the same as those encountered while getting to the last three cores. Once the final core is destroyed, the fort collapses.
Throughout the whole fort, the mechanics of Smash Run are in effect; if you get sent flying with enough damage into an area off the game screen, you will get KO'd. Enemies in the fort are shadowy versions of Geemers, Stalfos, Lethiniums and Plasma Wisps. Players can skip past the Stalfos and Geemers but should kill any Lethiniums and Plasma Wisps they encounter as they will be the units most likely to deal large amounts of damage while traversing the fortress.
The last form is a rainbow-colored orb resembling the series symbol, which must be knocked out of the blast-zone. It takes a while to launch it out of the arena, and using high power/knockback moves are recommended to launch it out of the blast-zone faster. The percentage to knock it out is usually at 140%+ (a short scene occurs when you defeat it). The Master Core does not fight back for a short duration, but if it is not knocked out of the blast-zone quickly enough, it will launch waves at the player which causes an instant KO if caught in the waves. If the player somehow manages to avoid them, the core will give up and explode.
Although the time limit freezes during this stage, essentially there is a limit due to the core's instant K.O attacks if not destroyed in time.
This sinister being will appear from Crazy Hand. A bunch of black objects, called "Swarm," will gather and mimic a variety of shapes, changing its attack pattern with each new form. After taking enough damage, it will reveal even more forms. Defeat the Swarm's forms to reveal its true core.
- Master Core's humanoid design resembles a Shaydas, but without the blades and only having one head.
- Master Core's wave attack resembles Tabuu's Off Wave attack.
- In a part of Master Core's music, it will play a morse code, which translates to Master Core. 
- There is a new theory that Master Core is the orb from the center of the large platform in the Battlefield in Melee. The Master Core form is a rainbow-colored orb that attacks with a massive swarm; in Multi-Man Melee, the Battlefield would be where a swarm of Fighting Wire Frames were encountered; this orb is also found in Super Smash Bros. Brawl's Battlefield stage, if one manipulates the camera look below the stage. In Super Smash Bros. for Nintendo 3DS and Wii U, this orb is not found in the Battlefield stage, indicating that the orb is Master Core joining the battle for the first time. See here for the video on the theory.
- Master Core's Swarm bears a huge resemblance to the Dark Matter from Kirby's Dreamland 2 and 3.
- Master Core's panel changes each time the player defeats its forms.
- If the player is controlling as either Link or Toon Link and reach the Master Shadow form, their shadow forms will be briefly seen in a pose which looks similar to their poses in the T-pose glitch.