The Dark Emperor (全てのやみの王, King of All Darkness) is a boss from the StreetPass Mii Plaza universe.
The Dark Emperor is the final boss of the Secret Quest of Find Mii II (also known as StreetPass Quest II) in the Nintendo 3DS application StreetPass Mii Plaza. He is a more powerful version of the Dark Lord, and his goal is to consume all light and plunge the world into darkness. He appears at the top of the Castle of Darkness - Roof level as the third and last boss, kidnapping the player's royal Mii after the armored archfiend and ultimate ghost are defeated. At the start of the battle, the Dark Emperor spews level-down gas to weaken the player's party of Mii heroes. Defeating the Dark Emperor finally frees the player's royal Mii and rewards the player with a treasure chest containing an accessory and/or golden key.
As a boss
The Dark Emperor appears as a boss on the Find Mii stage, and uses magic to increase or decrease a player's attack, defense, special attack, speed, or all four aforementioned stats. Red magic increases stats while blue magic decreases stats. The Dark Emperor can also use purple magic to cause an earthquake that temporarily destroys either of the two platforms of the stage.
The Dark Emperor will occasionally fly into the foreground, causing collision damage to nearby fighters and attacking with his roars. While the Dark Emperor deals minor contact damage, colliding with the Dark Emperor when he flies into the foreground or while he roars deals high knockback, making him dangerous near the sides of the stage. However, he can be defeated, which stuns him and grants stat boosts to whoever lands the final blow. After defeat, he will fly back into the background before respawning after a period of time.
: This enemy brings chaos to the battlefield, employing three different attack patterns. He'll use magic to help—or hinder—fighters depending on their color. He will also smash platforms with a localized earthquake. Last, he'll directly strike fighters. Wait a sec... What a cheater—he's invincible when he's in the background!
As a boss
The Dark Emperor returns as a stage boss in Super Smash Bros. Ultimate, functioning the same as in for 3DS, with the exception of his stat boosts being picked at random instead of choosing based on the color of each fighters' costumes.
The Dark Emperor appears as a Legend-class spirit. He can be summoned by combining the cores of Prophet, Vaati, and Necrozma. Alternatively, he can be acquired through a Spirit Battle in Adventure Mode: World of Light's Sacred Land, where his spirit uses a Giant Ridley puppet fighter with super armor.