It's fantastic, a surreal mystery about an underground town maintained by an automated system of Dwemer technology - one that's uncomfortably focused on the inhabitants' ethics. The Forgotten City is so good that it got enhanced into its own standalone game on multiple platforms, but you can still get it for Skyrim. As the harbinger of an ancient prophecy (minus any special powers), you have to bring peace back to Falskaar - but nothing is ever as simple as it seems. Considering it's the size of a DLC and that it landed its creator a job at Bungie, the fact that it's on this list shouldn't be that surprising.
Those of you who have finished Skyrim should head immediately to Falskaar.
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