Enderal is a Total Conversion (TC) of The Elder Scrolls: Skyrim. While it does use Skyrim's engine. combat system (with some tweaks) and most art assets, Enderal takes places on an entirely different world with its own environment, story, factions, and so on. You could perhaps liken it as to what Fallout New Vegas was to Fallout.
And the best thing? Enderal is completely free. You do need to own Skyrim, but Enderal essentially gives you a free AAA title with some 20-60 hours of playtime, professional voice actors and a story that makes that of Skyrim look like something drawn up by six-year-olds.
I can warmly recommend the TC for all TES fans and RPG fans in general.