His name and existence is deduced by taking all the capital letters in the cryptograms from Rumble's Revenge, to form the message: "MY NAME IS BILL." Trillions of years prior to the events of the series, Bill Cipher originated from the second dimension.
Bill despised living there, describing it as a dimension of "flat minds in a flat world with flat dreams." He "liberated" his dimension by plunging it into burning chaos, along with everyone he'd ever known, including his own parents.
They left behind elaborate cave paintings about their encounter with the demon, including how to summon him, and more importantly, a warning never to read the incantation that would summon Bill aloud.
The valley was deemed a "cursed land" by the natives, who evacuated around 1000 AD.
He eventually took over a boiling, shifting intergalactic foam between dimensions: a lawless, unstable crawlspace known as the Nightmare Realm.
Bill either could not see Ford in the thirty years he spent lost in other dimensions, or didn't care to, as Ford would remark that they had not seen each other in many years.
When Modoc learned of the prophecy that foretold of an apocalyptic event that would stem from interactions with Bill Cipher, he lit himself on fire in an effort to avert it.
The natives eventually discovered a way to defeat Bill by using a zodiac with ten symbols.
Among his targets were the natives who lived in what would become the town of Gravity Falls, Oregon.
Bill once asked a local shaman named Modoc to build an interdimensional gateway to the Nightmare Realm, but the result was made out of twigs.