( First things first. Unless you like playing with your game's gamma at the highest setting or have your monitor's brightness at maximum, I would suggest downloading a brighter version of this map. Otherwise, the mission is quite literally pitch black. There are no other differences between the two versions except the brightness.
https://thiefmissions.com/m/UnseenBright )

The Unseen is not exactly your typical Thief mission. Somehow it reminded me of stealth missions from those action/adventure games of the mid-to-late nineties.

You begin with absolutely no equipment and are tasked with navigating the environment by avoiding enemies, through acrobatics, and a couple of simple puzzles involving pushing hidden buttons. Starting without weapons certainly adds to the feeling of dread and vulnerability, yet the downside is that you progress by trial and error. All rooms have at least one enemy patrolling them and, with no means of defending yourself (mission does not forbid killing), even the slightest mistake means reloading the savegame. You quickly come across broadhead arrows, but your joy would be short-lived as the mission introduces an invisible (and quick-moving!) enemy next.

The Unseen was built for the NewDark "64 Cubed" Contest and it is impossible not to be amazed at how many various themes the author managed to utilize in such a small space. You've got your prison block, caverns, catacombs and mausoleum. And they are all just simply gorgeous looking.

The story has nothing to do with Thief games and involves the unnamed protagonist being captured on his way to the hollow Earth kingdom of Agartha. During the course of the mission names from Norse mythology and Lovecraftian universe gets their mention as well. Map is short and story never goes anywhere, so all that namesdropping ends up feeling of a bit of fluff. But you can't deny that it adds to the atmosphere of mystery that the mission has.