Enderal: Forgotten Stories is out now on Steam, making it the first Skyrim mod to launch as a (mostly) standalone game with achievements, cloud save support and even its own mod workshop. Set in its own, somewhat darker and grittier fantasy universe, Enderal is a well-known total conversion mod from SureAI, with this free Steam release being an expanded and polished edition. You'll need the original version of Skyrim, but otherwise it can be installed as a totally separate game. Sadly, if you only have Skyrim: Special Edition, you'll need to grab the original version too.
SureAI being a German team (though they've done a good job on localisation and English voice-acting), it feels like some of their influences lie in more number-crunchy and unforgiving European RPGs such as the Gothic series or Drakensang. Progression is a little more stat-based and old-school in Enderal than any modern Elder Scrolls game. Magic here is more complex, and casting too many spells afflicts you with 'arcane fever', and there's no level scaling. Some areas will just be too tough for you to attempt - return later with better gear and skills and you may stand a chance.
For those who really like their RPGs brutal and uncompromising, SureAI even reached out to modder "Ixion XVII" and let them poke around the Forgotten Stories version pre-release. The end result is this gameplay overhaul mod. While it broadly makes the game harder, it also adds new potion types, spells and adds new special attacks and abilities to monsters. Combat in general is faster, too, with both you and enemies taking more damage. There's a lot of other little changes, and you can read about the full list here on the mod's Nexus page. Good luck if you play it that way.
Enderal: Forgotten Stories is out now on Steam and free if you own the original version of Skyrim. If you don't own it, you can snag it here for £10/15/$15. It's a prerequisite for many Skyrim mods that haven't been adapted to the Special Edition.