From the Super Mario Wiki
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Generators are a type of enemy appearing in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS, appearing in the Subspace Emissary and Smash Run modes respectively and taking the appearance of a black and purple portal. They do not attack, instead spawning enemies, to attack the player; they mostly spawn Mites, though in Super Smash Bros. for Nintendo 3DS they may occasionally spawn different enemies. In Super Smash Bros. Brawl they do not stop spawning enemies until they are defeated, but in Super Smash Bros. for Nintendo 3DS they disappear after a set time, and defeating them before this gives the player stat boosts; they give more stat boosts the sooner they are defeated.
Super Smash Bros. for Nintendo 3DS Trophy information