The Hand Monstrosity's history is unknown, like many of Bill's Henchmaniacs, but considering it never gone to Gravity Falls when Bill unleashed Weirdmageddon, it's highly likely it's not an interdimensional criminal and is a resident of the Nightmare Realm that was tamed by Bill, but this is open to debate.
Before the start of the series, Ford got trapped in the Nightmare Realm, after Stan accidentally pushed him. Ford crossed Bill and he shouted: "LOOK WHO DECIDED TO PAY ME A VISIT! CARE FOR A GAME OF INTERGALACTIC CHESS? THIS TIME, YOU'RE THE PAWN!". Bill snapped his fingers and this monster howled and started to chase Ford, but he managed to escape it by hiding behind an asteroid. After it's failure, Bill screamed to his Henchmaniacs: "SIXER WANTS TO PLAY HIDE-AND-SEEK! FIRST ONE TO FIND HIM AND BRING HIM TO ME GETS THEIR OWN GALAXY." Ford escaped them by hiding in a cave, meeting the space miners that got trapped.
The Hand Monstrosity makes it's physical appearance during the tale "Don't Dimension It!" from "Gravity Falls: Lost Legends", after Weirdmageddon, a bunch of residual rifts appeared across the town and Ford and his family gone to seal them. Mabel stood very close of the rift and this monster snatched her, dragging her to the Nightmare Realm. Mabel managed to outrun him, but she fell through many dimensions until she reached MAB-3L. The Hand Monstrosity doesn't appear for the remainder of the chapter.
Ford describes this creature as: "a 60-foot-tall ball of fingers and teeth," and has a howl like the one of a humpback whale. The creature is itself is a massive hand-like monster with a mouth in it's palm, similar to an octopus and posses 7 fingers. The Hand Monstrosity is very large, if we take Ford's word, it should be at least 60 foot tall, it seems to be 5 times large the size of Mabel.
- It's highly likely it's not a Hench maniac and is instead a resident of the Nightmare Realm that was tamed by Bill, considering how he behaves.
- It never went to Gravity Falls during Weirdmageddon.
- In Weirdmageddon exist creatures such as the Giant Ear, The Horrifying Sweaty One-Armed Monstrosity and the Creature with 88 different faces, but their relationship with him is unknown.