The Master Core is a character who first appeared in Super Smash Bros. for Nintendo 3DS and Wii U. He is the final boss of Classic Mode if the difficulty is set to intensity 6 or higher. After Master Hand loses a portion of his stamina, Crazy Hand leaves, engulfed in flames, and Master Hand's glove tears open, and the Master Core spills out.
Master Core hides his true form using a swarm of black particles. His first phase portrays him as a giant entity whose head bears a glowing, blue-white face. This is the Master Giant. During his second phase, the Master Core becomes a scorpion-like creature known as the Master Beast, whose mouth contains an orange void. His third phase turns him into the Master Sabers, whose body consists of five swords with the largest one being the "head". His fourth phase allows him to mimic his opponent.
During his fifth phase, which is skipped in the 3DS version (and the Wii U version if the intensity is set to a level below 8), the Master Core creates a giant black fortress whose design parodies the human digestive system.
Once the swarm is dispelled, the Master Core's true form is revealed to be a golden orb whose appearance is similar to a Smash Ball.
The Master Core appears after Master Hand is defeated in Classic Mode at intensity 6 or higher. When he enters the scene, he tears his way out of Master Hand's glove, killing the deity as the background turns into a fiery, swirling warp field.
Phase 1: Master Giant
During his first phase, the Master Core appears as a giant humanoid figure. Only his head can be attacked during this phase. His attacks include the following:
- Vacuum Breath: The Master Giant's mouth opens and inhales, and four orbs fly in from one side to knock the player into his maw. Getting caught means massive damage.
- Grab and Feast: Two hands fly across the stage. Getting caught has the same effect as getting caught in the giant's mouth.
- Stage Bearer: The Master Giant lifts up the stage, causing the player to get hit by some white orbs.
- Bomb Bite: The giant slams his head into the stage, unleashing four purple balls that explode like an X-Bomb would.
- Sooper Strike: A horizontal lightning bolt goes across the stage. It can penetrate through shields. This is telegraphed by the giant moving his arm and can be avoided with a dodge.
- Scream Blast: A scream issues from the giant's mouth, dealing damage to characters touching his head and causing a bit of set knockback that is not enough to score a KO.
Phase 2: Master Beast
During his second phase, the Master Core becomes a giant scorpion-like creature. His entire body can be damaged during this phase.
- Thunder Flash: The Master Beast's body crackles with lightning, followed by a thunderbolt hitting his back.
- Hungry Beast: The Master Beast jumps into the background and tries to bite the player.
- Crush Rush: The Master Beast jumps into the air and attempts to crush the player, causing the stage to tilt if his attack misses.
- Thorn Blast: The Master Beast spawns spikes on his back. Shadows will follow the players position. When they reach where the player is, thorns will pop up, but can be dodged or shielded.
Phase 3: Master Sabres
During his third phase, the Master Core transforms into a set of five swords. The swords vary in their size, but one is larger and commands all of them. The swords share their health bar.
- Basic Swipe: Master Edges swipes at the player.
- Fury Slash: Master Edges follows the player before furiously slashing at the player with his swords.
- Boomerang Storm: Master Edges swipes. His swiping launches four projectiles which boomerang back to him.
- Boomerang Storm, Version 2: Master Edges swipes, launching two large projectiles that reverse course back to his largest sword.
- Quintiplex Sword Slashes: Master Edges's swords space themselves across the stage, before slashing their target. The largest sword strikes last, and deals a lot of damage and knockback on a hit, but their attacks can be shielded.
Phase 4: Master Shadow
The Master Core will impersonate the player during his fourth phase.
- Clone War: The Master Core will glow blue during two-player classic mode. His attacks in this form are the same as those of the character who is fighting him. The Master Core will also copy his opponent's gear. Once his stamina is drained in this form, the Master Core will either reveal his true form, or flee into a black fortress, the latter of which occurs in the game's Wii U remake at intensity 8 or higher.
Phase 5: Master Fortress
This giant fortress appears during the Master Core's fifth phase in the Wii U version of Super Smash Bros. 4 on intensity 8 or higher. It is based off a human digestive system. The player must venture through the fortress while searching for four cores while avoiding the guards. The first one is available immediately, possessing few traps and guards. The second core is guarded rather well, and Danger Zones appear, dealing 25 acid damage to their target and causing a death if that target started at 100% on their damage meter before the hit. Once the second core is destroyed, the background music becomes frantic and the player is literally kicked out of the fortress and needs to head to the fortress's other side to destroy the third core. This causes the fortress to crumble slightly, revealing the path to the fourth core. However, this path has lots of guards, traps, and shifting platforms which would make navigation trickier. The fourth core's destruction will destroy the fortress, causing Master Core's true form to be revealed.
During his Final Phase, players must knock Master Core's round body off the stage. If they are slow, the Master Core will rise and launch shockwaves that can cause a one-hit KO. After the fifth shockwave, the core's body becomes unstable leading to his destruction, though the core's dying explosion is harmless more than 2 metres away. Close contact, though, causes a death for the player.
- The Master Core is the second boss in the Super Smash Bros. franchise who can manipulate the stage's layout, following Meta-Ridley. However, he only does this in his Giant, Beast, and Fortress phases.
- In a small part of his theme, the Master Core's full can somewhat be heard in Morse code.
- The Master Core's Shadow and Fortress forms do not appear in Super Smash Bros. Ultimate at all, although his Giant, Beast, Sabres, and true forms do as Spirits.
- The Master Core's Swarm greatly resembles the Shadow Bugs from Super Smash Bros. Brawl and his Beast form resembles the Chaos Kin's burnt form from Kid Icarus: Uprising.
- The Master Core's Sabres form's swords can leave a skull image in their wake when the boss uses his Basic Swipe attack.