"It's just really hard to believe."
So the next morning they can make their way to the temple, Ruby on a conjured horse and Erandur on a real one that he owns.
"So, what were the missing details?"
He sighs.
"The temple hasn't been abandoned. Its members started stealing the memories of a group of orc raiders, decades ago, and they invaded it in retaliation; when it became clear to the cultists they would lose, they released a powerful magic gas called the Miasma, which put everyone in the tower to sleep and magically sealed most of it off. I expect that once we unseal it, the Miasma will dissipate, and all of them will wake up."
"Wait, decades? I guess if all of them were elves, but then wouldn't they die of hunger?"
"The Miasma prevents aging—or rather, it keeps the people affected in a state of near-stasis, immune to most of the ravages of time."
"Yes. The soul isn't meant to be in stasis for decades, and unless one's soul is unusually resilient, they can become severely damaged, sometimes entirely losing their mind. Sometimes even dying."
"Some. There was a fort near Windhelm called Morvunskar which I, ah, liberated from a group of necromancers."
Man if so that's. That's too many. Too many spirits from outside Mundus taking an interest in him.
"How do you know all this?"
"I have dealt with this group of cultists before, and have witnessed their practices."
The temple is a tower overlooking Dawnstar from the east on a mountaintop. After dispatching an Ice Wraith that had made its home there and checking that there aren't any other surprises around they walk into the temple.
All of it—except, that is, for the Sanguine Rose, the staff Sanguine gave Ruby.
He starts swearing profusely.
"...this is not a security measure I'm familiar with," Erandur says, after he's recovered his balance and is looking around. "It's ingenious, stripping invaders of their equipment, but I have no idea how it could be accomplished."
Ruby leans over to grab the staff and sigh. "My turn to disclose some not exactly secrets: the Daedric Prince of Hedonism, Sanguine, has taken a liking to me, and has said that he wanted to see what I'd do, in the future. This—" Gesture at staff. "—is a staff he gave me. I am starting to rethink my life choices, here."
"—no! Absolutely not. Sanguine's, like, a cool guy, I guess, but I am not a cultist. She just... thinks I'm hot, I guess."
"The Daedric Prince of Hedonism... thinks you're hot. In my experience, drawing the attention of Daedra Lords is not wise."
"I didn't have much of a choice, honestly, he sent me on a merry chase around Skyrim and then decided she was going to keep me."