It could be part of a downloadable content (DLC). Check the official game website or the platform you bought the game from (like Steam) for any available DLC related to the alternate story. Sometimes, there are also special editions of the game that come with extra story content built - in.
You might be able to access it through in - game unlocks. Maybe by completing certain difficult quests or achieving high levels in the main game.
If it's a mod - made alternate story, you'll need to look for it on modding communities. However, be careful when installing mods as they can sometimes cause compatibility issues or glitches. Make sure to follow the installation instructions carefully and only download from reliable sources. Also, keep in mind that using mods might void your game's warranty or violate the game's terms of service in some cases, especially if they involve unlicensed or inappropriate content.