Fallout 4‘s Far Harbor DLC introduces a number of mysterious factions across the foggy island map. There’s the sour harbor folk, the secretive synths of Acadia, and the radiation-worshiping cultists of the Children of Atom. The faction has appeared in other games previously, and even shows up in the Glowing Sea of Fallout 4, but they are present in Far Harbor with greater strength than ever, based in an old nuclear submarine.

https://screenrant.com/fallout-4-far-harbor-confessor-martin-children-atom/