The Lich wants to cause chaos in the land of Ooo, so he recruits Lemongrab and HunsonAbadeer to capture the Hero's flag: the symbol of all the good in the world.

If the Hero's flag is captured, chaos can reign supreme under the power of evil. However, for the Lich to be present in the land of Ooo, he had to conjure the flag of

Pure Evil, else he can't manifest himself as a physical form.

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The Lich must have the flag of Pure Evil near him at all times because if it is

captured then he simply disappears from the Land of Ooo. Bubblegum princess is made aware of the three evil characters coming together in the name of chaos, and recruits Finn and Jake to help her defend the Hero's flag, and capture the flag of Pure Evil, at all costs