Weirdmageddon (event)

Weirdmageddon or Oddpocalypse is a term used by Ford to describe the apocalyptic result of the interdimensional rift breaking. In "Dipper and Mabel vs. the Future," this event is triggered by the liberation of an interdimensional rift, which allowed the demon Bill Cipher and his "friends", to escape the Nightmare Realm and flood into Gravity Falls. In this event, Bill will summon creatures from the nightmare realm to have an insane party and wreak havoc upon the entire town.

History
This event is foreshadowed loosely in "Dreamscaperers" by Bill Cipher, and is foreshadowed multiple times throughout the first half of season 2.

Stanford Pines first refers to it as the name Weirdmageddon in "Dipper and Mabel vs. the Future." Later in the episode, Bill possesses Blendin Blandin to get the interdimensional rift from Mabel Pines. He smashes it to start the event, which formed an inter dimensional portal in the sky, and the episode ends shortly after with his chaos just starting.

Trivia

 * This event is a portmanteau of "weird" and "Armageddon."
 * Bill Cipher has obtained a physical form for Gravity Falls' reality.
 * Gideon Gleeful has decided to rebel against Bill Cipher to gain Mabel's love.