https://wccftech.com/fallout-4-story-mod-sakhalin-released/

Sakhalin is a recently released Fallout 4 mod focused entirely on delivering new content. It was made by developer NowSky in one month as a side project; the team previously created the Gothic Orpheus mod for The Elder Scrolls V: Skyrim, as well as 20 Mount and Blade: Warband custom maps and an expansion. They’re now working on a brand new standalone open world game called Trail of Ayash, focused on the native American people.

Sakhalin, on the other hand, is set in an alternate Fallout universe as described below. It packs a new island, new quests, factions, monsters and even new interiors. In order to access it, you’ll need all Fallout 4 DLCs installed; once in the game, head to the Soviet Port located in the southeastern part of the Commonwealth map.