The Forgotten City was made by Nick Pearce, an Australian modder. It's ridiculous.
It's a murder mystery essentially, set in an underground city. The production values, for a mod, are completely ridiculous. It's out now, you can play it right now and if you're a fan of Skyrim you probably should.
Stevivor is currently running a great interview with the creator of the mod. He mentioned that he spent over 1700 hours creating it. My favourite part of the interview: "I’ve spent over 1700 hours on this project, so if 283 people play it for 6 hours each, then that’s a work to entertainment ratio of 1:1 – and I’d call that a win."
I have a suspicion he'll hit that 1:1 ration fairly easily.
The Forgotten City is a pretty big deal. It features 18 voice actors, delivering over thousands of lines of dialogue and tried to focus on creating =non-linear missions that veer away from the usual 'go to this place and deliver this item' fetch quests.
You can download the mod here.