And where do you think you're going? Demon

When Mason D Cipher was born the flat dimension was seeing its destruction. Will Cipher, Mason's mother in a haste sent the newborn into the future and to another dimension with a note explaining his situation before Will left himself to avoid the wrath that his husband had created after getting angry.

The dimension in which he was sent? The 3rd dimension obviously, not only that, the baby appeared in Stanford Pines laboratory while Stanley was trying to find a way to save his brother.

Bill Cipher doesn't even know he has a son, he believes that he accidently killed it in his rampage in the flat dimension. Same goes for his 'wife'.

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In the present it's 2 years after Bill was defeated, Mabel and Dipper are cleaning up in their Grunkle Ford's lab, however the portal starts to work and begins pulling the twins in, Dipper manages to shove his sister out of the way in time and instead only he gets pulled in.

Let's see how things work out~