The Long Lost Mixamajig was a mythical object created by the Nixels to fool the Mixels into going on an epic quest to find it. This ended up congregating them all in one place to be easily exterminated by King Nixel.
It was described by the Maximum Mixel shadow puppet as something that would grant its possessor their wildest Mixing dreams. The Mixamajig Key was needed to unlock it. In reality, it ended up being, ironically, a simple eggbeater which broke at first use.