Commonwealth
The Commonwealth of Massachusetts, more commonly referred to as just "The Commonwealth," is a geographic region consisting of the former US State of Massachusetts, notably including the former state capitol of Boston. It is the primary game world for the base game of Fallout 4.
Background
In 2087, the world was nearly destroyed in a global nuclear conflict known as The Great War, in which thousands of nuclear missiles were launched around the world. Much of civilization was destroyed and the surface was left irradiated, filled with deadly mutated creatures and radiation. However, humanity would persist and try to rebuild after the war, even forming entire settlements such as Diamond City.
The destruction and radiation from The Great War caused many additional issues. Due to Native American burial grounds being destroyed by the bombs, the angry spirits of the Wendigo were released across the Commonwealth.
Some time in 2275 or prior, The Order inside the town of Whispering Hills would sacrifice Alyssa as part of The Ritual. However, due to missing two of the three required sacrifices, the Ritual failed and did not bind the ungodliness. As a result, the town would be transformed into a monster-filled nightmare. An unknown higher power would start to "feed" off the citizens of the town, making nearly all the residents slowly disappear without a trace. Once it began to run out of people in Whispering Hills, it started to open passages to the Commonwealth to lure in more people to feed on. It is unknown how many passages it opened, but it opened at least one former (now closed) passage in the Parsons State Insane Asylum, and an active one remains at the bottom of the Dunwich Borers excavation site.
After the Ritual, many of the horrors from Whispering Hills would invade the Commonwealth. A thick fog permanently enveloped all of the Commonwealth. At seemingly random times, it would be engulfed in darkness in Otherworld events where swarms of monsters would invade from Hell. Some of these monsters, such as the Sirenhead, would stay within the Commonwealth in certain locations even outside of Otherworld events. Feral ghouls would transformed or replaced by other monsters, with many Eyeless Gnashers being found in the place of regular ghouls across the Commonwealth and Bubblehead Nurses being found replacing ghouls in the hospitals of the Commonwealth.
All of these effects also applied to the nearby locations of Far Harbor and Nuka World; it is, however, unknown just how far the effects reached, but it is likely not too far outside of the Commonwealth due to Harry Manson only referring to the effects in regards to the Commonwealth instead of the greater United States.
In 2276 or 2277, Shaun would be kidnapped from Vault 111 in the Commonwealth and brought to the town of Whispering Hills by The Order, with the help of The Institute, to be part of a prophesied unholy marriage. Before the Order took Shaun, the Institute used Shaun's unirradiated DNA to create their Generation 3 model of Synthetic Humans.