Hmmm, no I can't say that I have heard of any Mer liches, and while I haven't heard specifically of Deepholme, I have heard of the Atmoran's use of hidden prisons. Fascinating. The night of tears does come to mind though. It tells the accounts of the snow elves invasion of the Nedic capital of Saarthal. Nobody is entirely sure why they invaded. The Mer had more or less managed to live in peace with early man and Skyrim is the only place that outright war ensued. Well admittedly it's just a theory, but if the Atmorans had captured an influential Snow Elf and held them prisoner, the elves certainly would mount efforts to get them back. If they held this individual in Deepholme and the Elves attacked Saarthal though. That would indicate that Deepholme was quite protected and a well kept secret if that were the case.
The quest to unlock Deepholme will trigger when you go to Deepholme after having completed the heist quest 'Shadows of One's Past' and set up the Explorer's Society. After 'The Vaults of Deepholme' quest has finished, Deepholme itself will become available as an off-site storage for powerful artifacts and magic items, while replicas can stay on display in the museum without being at risk of another heist.
In order to enter after the quest, the Totemic Key is required to lower the gate at the entrance.
Most of the items will only display when placing them in the appropriate sorting chests. Once displayed they can either be taken from their display spots or directly back from the chests.
To the north, on the far side of the large hall, is the Daedric room on a platform above a lava pit secure behind multiple gates and a draw bridge. Items must be placed in the "Daedric Relics" chest at the center of the room.
The upper three rooms on the west side of the hall feature the Weapon Displays. All three rooms contain a weapon chest, the chests are linked together and display the weapons in whichever of the three rooms the weapon goes to.
The upper three rooms on the east side feature the Armor Displays with an Armor chest in each of them. Again, the chests are linked together and display the items across the three rooms.
In the second room of the Armor Displays a Jewelry Display case is found, a small "Jewelry Strongbox" serves as the auto-sorter for the rings and necklaces.
The lower first room to the west contains displays for some special items that don't fit into the other categories. They must be manually placed.
The lower third room to the west features a display for the three Elder Scrolls. They must be manually placed.
The lower first room to the east is currently empty.
The lower second room to the east features displays for the Black Books and the Mythic Dawn Commentaries. They must be manually placed.
The lower third room to the east features a large display for the Dragonpriest masks. Items must be placed in the "Stone Chest" at the display.
- Although Deepholme is a perfect protection against further raids of thieves, neglecting to transport all items to Deepholme will have no in-game effect.
- It is not possible to fill all displays without console commands. The Deepholme displays only accept originals, and some items are either mutually exclusive or are turned in during quests.