Bill Cipher the SandWing

Bill Cipher is a triangular dream demon that came from the second dimension. He entered Pyrrhia in 5098 A.S. and cause mass destruction, destroying Pantala and wiping out all of the tribes there. He was stopped by ten powerful animi, who were reincarnated into new forms to stop Bill over 8000 years later.

History
Bill Cipher originated from the second dimension, he despised it, describing ita as a dimension of "flat minds in a flat world with flat dreams." He learnt how to access the higher dimensions, becoming a powerful animus magic user. He used his magic to liberate his dimension, plunging it into burning chaos, killing everyone, including his parents. After destroying his home dimension, he left, taking over the boiling, shifting intergalactic foam between dimensions: a lawless, unstable crawlspace called the Nightmare Realm. He ruled over all beings living there using his powerful animus magic. However, due to the Nightmare Realm's lawlessness and lack of any consistent physics or rules, it was fated to collapse into itself. He used his animus magic to see into the third dimension, learning of a prophecy told by a powerful NightWing seer. It stated that he would merge the Nightmare Realm with the third dimension.

He started to weaken the dimensional walls with his magic, the dimensional walls being the weakest in an area which would eventually be known as Gravity Falls. He was eventually able to weaken the dimensional walls enough to enter the third dimension. He took over the body of a powerful SandWing animus, using both of their animus magic combined to open a large portal on Pantala, causing mass destruction. Ten powerful animi came together to take down Bill. Their fight led to the sinking of Pantala and the deaths of its inhabitants. Bill's left eye was ripped out and he was trapped in the Nightmare Realm, with most of his work undone. However, the dimensional wall in Gravity Falls still being weak.

He was stuck in the Nightmare Realm, not being able to contact the third dimension directly due to the loss of his physical form and the spell put on the Nightmare Realm by the ten animi. He quickly found out about the weakness in Gravity Falls and was able to access the dreams of the inhabitants. This caused random weaknesses all around the world. He tried to get a shaman in Gravity Falls to build an interdimensional gateway to the Nightmare Realm, but it was made out of twigs. Modoc learnt of the prophecy that foretold the destruction that would come from Bill, so he lit himself on fire in an attempt to avert it.

Appearance
Bill's true form is a yellow, one eyed equilateral triangle. His one eye is rimmed with four short black lashes on top and bottom. Even though he has legs, he moves around mostly by floating. He wears a long top hat and a bow tie. His body has a brick pattern on it, stopping at his bow tie.

When appearing to dragons, he takes the form of a SandWing with one eye ripped out. A red x is marked over his ripped out eye. Next to his ripped out eye is his prophecy mark, which is his eye from his true form.

http://gravityfalls.wikia.com/wiki/Bill_Cipher

http://gravityfalls.wikia.com/wiki/Zodiac