So you got any more details around what happened with Stan and Ford in their youth? -eyes-
Ok so I actually have all of the kids Bill/Beldam (who I'm just calling 'Cipher') lured and killed mapped out but to focus on Stan and Ford:
In the early 80s, the Pines went vacationing to Oregon. Filbrick Pines was a part of a fishing lodge with connections all over the country, and was randomly picked to stay in a cabin there for free. Filbrick probably wouldn't have stood for it if it wasn't free. It was stormy and hot in their town, too, with the few visitors Glass Shard got disinclined to go wandering around and stopping by pawn shops, so Filbrick conceded that having the shop closed for a week or two while the stormy weather and heatwave passed wouldn't do much. And maybe even an implacable brick of a man like him wanted some time off too.
So to Gravity Falls the Pines go. Filbrick, Caryn, their eldest son Shermie, and Stanford and Stanley.
Stan and Ford are left to run around like they usually are, and explore the woods together while their father and brother fish on Gravity Falls' placid lake. They find an old bricked up well and when they attempt to look down it, they fall in instead.
But instead of tumbling down into well water or cracking their heads open, the tunnel distorts and glows and they're spit out into a strange version of reality. Once they right themselves, they hear something coming.
It's their father. Except--he's got buttons in place of his eyes.
Ford is fascinated. Stan is grossed out but intrigued.
The man that introduces himself as their 'Other Father' is warm and kind and wants to know all about them. He picks Stan up and puts him on his shoulders and takes Ford's hand and doesn't scold Ford for shying away for a second. He answers every question they have with explanations, not "Because I say so." The Other Mother they meet is similarly distinct from their mother. She doesn't lie as constantly as their mom. It's easier to believe her when she says "I love you." Their Other Brother is always willing to play games and entertain them.
Stan and Ford have an absolutely wonderful time. But they don't want to replace their eyes with buttons. Their other parents keep pushing and pushing and pushing, and eventually the mask slips and Cipher, the real puppeteer here, attempts to issue challenges to keep them in the Other World before they can run away. He traps Stan to get Ford to accept the challenge, as Ford was the one he was more interested in anyway.
A lot goes down, but the important thing is this: Stan helps Ford escape back to the real world, blocking off the one entrance they know of (the well) in the process. Stan is stuck with Cipher in the Other World.
Cipher keeps trying to get Stan to accept the buttons, but Stan's as stubborn as a mule. He never takes on the terms of any deal or game.
So, in a fit of frustration, Cipher just steals Stan's eyes without any agreement and gives him the button eyes anyway.
But the thing is, the magic of the Other World and Cipher's ability to feed on the children he lures is very particular. The child must agree to give up their eyes for the buttons. That's why Cipher makes promises of a perfect life, that's why when the lies fall apart he works in terms of games with losing conditions and deals and trades. He needs the child to give up.
And if he steals a child's eyes when they haven't given up, he can't eat them.
So Stan's stuck in the Other World, not quite dead and not quite alive. Cipher has control of him as a puppet, but Cipher didn't want a puppet. He wanted a meal! And he lost two meals in one go.
But then. Decades after Ford attempts to explain where Stan vanished to are brushed off, decades after the woods were searched and nothing found and the Pines were forced to pack up and go back home with one less son, decades after Ford returned to Gravity Falls, desperate to figure out if there really is an Other World or if it was the product of a child unable to understand what really happened like everyone else says, more Pines children come to the cabin.
Cipher keeps an eye on Mabel Pines. Her and her unbridled energy and curious mind are exactly what drew him to Ford Pines (and, incidentally, Stan Pines) around thirty years ago.
This time he won't let the twin in. He can learn from his mistakes.
He can even make use of his mistakes--why bother making a Stanford puppet when he has a near-perfect copy right here? A copy that he can torment by forcing him to lure in his own niece to be eaten.
And he will get her this time. She's a perfect mark. Lonely and bored and gullible and not talking to her twin very much.