the dunwich building full of ghouls worshiping an obelisk :tops:
I said a vault, not a building.
Did a research and it was vault 106 I was talking about.
The source of the hallucinations in vault 106 was a strong psychoactive drug pumped in the air filtration system.
Any hallucination in Skyrim could be cause by magic "traps" and strong chemical gas made by alchemists or so.
That could fit in.