"Mara's trial?"

"What is important here is not following a god, it is following her precepts. If the path of love and compassion is one in which I am no longer Mara's, then... I believe she would forgive me, and I believe she would want me to do it. The innocents who are being affected by the nightmares and those who might yet suffer at the hands of the cultists are worth more than my devotion or the salvation of my own soul."

...Ruby's tired of holding onto the suspicious frame of mind. He thinks, he really thinks, that Erandur has had enough opportunities to get rid of him, or work against him. Hells, if he had taken the side of his more lucid erstwhile companions during the earlier fight, Ruby doesn't think he would've been able to overcome them.

"I'll do it."

"No. Your suspicions are warranted, and I would not ask that you betray your principles for this. This is my project, and it is my sin to atone for. Your help has been invaluable, but I will not have you shoulder this burden."

"You've... proven yourself. I think it would be just as much a betrayal of my principles, to not believe the evidence of my senses.

"I... trust you."

"...very well. I will do my best to be worthy of your trust."

"And just in case, the Dremora is still bound to me, so if you do anything untoward you will have it to contend with."

Erandur cracks a smile at that. "Trust, but verify."

"Oh that's a useful phrase, I like it, I'm stealing it."

They can return to the alchemical lab and fetch the Torpor, then.

"It should be just a matter of imbibing it. I will hold aloft a memory, and I believe it will preferentially use that memory. When you find yourself on the other side of this barrier, will yourself awake, and you will be there."

"Sounds straightforward enough. Here's to hoping this works." And he drinks the potion.

Erandur is in the inner sanctum of the tower, standing by the altar where the Skull of Corruption can be found. Two other cultists are there with him.

"The orcs have reached the inner sanctum, Brother Varen," says one of them.

"We must hold; we cannot allow the Skull to fall into their hands," the other one replies.

"I share your conviction, Brother, but... no more than a handful of us remain. The orcs have us overpowered and outnumbered. You know what we must do, yes?"

"We have no choice," Erandur agrees. "We must release the Miasma."

He nods. "We have no alternative. We must put our faith in Lord Vaermina, that she will send her chosen to recover the Skull."

The one named Varen looks at Erandur. "Brother Casimir, you must activate the barrier and release the Miasma." Then he looks at the other one: "Brother Thorek, you and I must remain here and see to it that none reach our Lord's holiest artefact."

"To the death."

"I... I will not disappoint you. Farewell, my brothers. The will of Vaermina be done."

"The will of Vaermina be done," the other two agree.

The rest of the memory seems almost a blur, probably an artefact of Erandur himself not remembering it well. He walks past pockets of fighting, through rooms and up stairs and ramps, all the way up to just before where the barrier will be, in the future. An image of Lord Vaermina is engraved on the wall, there, and after sending a prayer up into the heavens, Erandur—Casimir—activates a complex spell hidden inside the image to release the Miasma into the whole tower and to activate the barriers in and around the temple.

...and, after a moment's hesitation, he decides to escape rather than stay inside with the other cultists.

But Ruby doesn't want to follow him out, so he wills himself awake before Casimir can cross the threshold of the barrier-to-be. It feels just like waking up from a daydream, like he was reliving a memory of his own in a moment's reverie, and he would almost believe it to be so, were it not for the fact that he does now find himself on the other side of the barrier.

He can feel the Miasma on this side, thick on the ground, start to take effect on him, but before it can do that he sends the pulse of magicka into the barrier which Erandur said should suffice to break it. It's meant to keep people out, not to keep anyone in, and thus is designed to be easy to take down from here.

It works as expected, and as the Miasma starts spilling out it stops affecting Ruby.

"Remarkable... Your body just vanished and then reappeared on the other side."