The Grand Cosmos.

[Vyrerus]

Yeah, it's the palace of the Elezen civ that was in Lakeland, the same one that had that big castle on the mount. The elezen that come out of the paintings that you fight are recreations of their best warriors, notably wearing dyed Deepshadow gear, hinting that these Elezens were responsible for the creation of that gear.

The ghosts are the Nu Mou's way of livening up the place, without being actually bothered by real people. In the same way they ensorcel brooms, bring sentience and humanoid shape to plants, and imbue paintings/statues with life.

As for the Warg gear, well, Wargs are an overworld enemy fought in Lakeland. So the sets are their hide, shaped into clothes, so perhaps old trophies from a time when Lakeland was far more rustic. Though, the way Wargs look in game versus how the sets look, don't quite match up, but ehh, close enough for Hydaelyn's work.

[Razard]

The creepy thing about the Warg gear is that it is hinted in a fate that the Wargs are transformed Elves.

Think about that for a second.