Online:Mistress of the Lake

Quick Walkthrough
# Talk to Herald Kixathi.
# Take wisp essences to the essence troughs.
# Find Apostle Marian.
# Free the disciples.
# Return to Marian.

Detailed Walkthrough

Herald Kixathi can either be found on the road leading southwest of the Halcyon Lake Wayshrine or immediately south of the Halcyon lake middle ruin.
:''"You heard the call, didn't you? You have been chosen to help stop the blasphemy in the ruins below, to stop the laws of nature from being broken.  I too was carried here on the tides of fate for this reason. We must work together."
:Are you a Dremora?
:"Yes. I serve Hermaeus Mora, the Lord of Fate and Knowledge. These ruins were once a shrine to my lord, a storehouse of secrets forbidden to mortals. The shrine is but a memory now, but an entity invades the ruins seeking to steal the secrets."
:What kind of an entity?
:"A spirit? A Daedra? This is part of what must be exposed.  Whatever it is, it has enslaved local villagers—calling them Rain Disciples. It taps their minds for its power. To what end I do not know, but I believe you can unmask its intentions."
:All right. What do I have to do?
:"Wisps serve the entity below, drawing the essences of the enslaved Rain Disciples and depositing these essences in troughs. Slay one of the wisps and take the essence it carries to the troughs. I believe this will expose the entity's intentions."''

Getting the quest from near the wayshrine leads you into a cave, where wisps and Rain Disciple enemies can be found. After killing enough Wisps to collect their essence, you can take it to the Essence Troughs, also surrounded by enemies. Each trough filled will prompt a voice urging the other Disciples to accomplish their mysterious goal. You will also hear a name, Apostle Marian.

Apostle Marian can be found via a path leading south of the cave complex. She explains that the followers of the Daedric Prince Hermaeus Mora have, unsurprisingly, been manipulating the Vestige into slowing down the Water Disciples' efforts to restore the lake with ancient magic. She pleads with you to rescue captured Rain Disciples and other villagers. 
:"Please stranger! We mean you no harm, yet you enter our domain with distrust, killing our disciples. My mistress says you have been misled - that you should help rather than hinder us in our endeavors."
:Who is your mistress?
:"Lorelia. She is a nereid. She is the water spirit of Lake Halcyon. Lorelia says that the Primeval Seekers have misled you. She is an element of nature, seeking only to foster life. She told me to explain her purposes here."
:What are her purposes here?
:"The landscape of Tamriel is ever-changing. The desert south of here is expanding and will someday overtake this area too. Lake Halycon has already begun to slowly dry up. Lorelia only seeks to prevent that, to keep the lake and the forest alive."
:How does she plan to do that?
:"These ruins hold ancient secrets and relics of power. With them, Lorelia can create a Water Stone - an everlasting source of water that can keep Lake Halycon alive for all eternity. Myself, the Rain Disciples, and the wisps aid her in this mission."
:'''The Primeval Seekers say you're breaking the laws of nature.
:"The cultists worship the greedy lord, Hermaeus Mora. They would hoard the secrets buried here; they do not wish us to make use of them. They will do anything. They have even been capturing our disciples and holding them prisoner."
:Why would they capture your disciples?
:"Just to hinder our efforts. That's why they sent you as well. I'm sorry you've been caught up in this. I won't ask you to join us, but surely you can see that our disciples have done no wrong. Would you be willing to free them as a favor?"
:I will free your people, but I promise nothing more.

They can be found in the set next of chambers, guarded by Watchers and Primeval Seekers: Disciple Cesamund, Disciple Dianette, Disciple Jeannelle, and Disciple Theophile. Jeanelle and Cesamund are on the lower levels while the other two are on the upper levels, and their dialogue hints at their current state.

When you return to Apostle Marian, you find Herald Kixathi instead waiting for you. She will be furious with what you have done.
:"You ignorant half-wit! By the Golden Eye, what in Oblivion are you doing? Setting those poor, ensorcelled fools free? Don't you realize we were trying to help them?"
:How was capturing them trying to help them?
:"Don't you see what's going on here? I followed you before and overheard your conversation with that mad apostle. It's clear that this nereid learned one of the most dangerous secrets about the natural order and now twists it to her advantage."
:What dangerous secret is that?
:"No point hiding it now … when a mortal dies, where do you think their memories go? Don't bother guessing. I'll tell you—they go into the water. They become water. All the memories of Tamriel's history are stored in its waters."
:How do memories turn into water?
:"The details and implications of this secret are mind-spinning and would require a lifetime of study and contemplation. The relevant fact here is that the nereid is stealing her disciples' memories to create this Water Stone."
:So that's why they can't remember anything?
:"Exactly. And it seems they never will. Their memories are lost to them forever. The Lord of Knowledge abhors this transgression. The nereid must not be allowed to continue in this endeavor, nor can she be permitted to create a Water Stone."
:What's so bad about the Water Stone?
:"The secrets to creating a Water Stone are forbidden. The natural progression is for the landscape in this area to evolve into a desert. The nereid's not fighting for nature. She's fighting for self-preservation, and at her disciples' expense."
:So you think the nereid should be killed?
:"There is no other way for this to end. The nereid has stolen forbidden secrets and committed crimes against nature and humanity. Now that you understand, will you repent for changing loyalties? Will you aid me in destroying this aberration?"

If you are not yet ready to make a choice and continue further across the water pass Apostle Marian will be standing there waiting for you.
:"There you are! Lorelia was starting to worry. She said those cultist were trying to mislead you again. She worried that you would turn against us. I knew it was all right though. Why would you possible oppose Lorelia?"
:Do you realize that she's stealing all her disciples' memories?
:"Ha! Of course! It's sacrifice I'm willing to make. She showed me what would happen if if the lake were to dry up. She showed me how so much life would be lost. How could I refuse to give up some silly memories when so much is at stake?"
:The other disciples made this sacrifice willingly too?
:"Some did. Some didn't. You must understand though when people come wandering around Lorelia's lair, what is she to do? She's a nereid! When a fly gets stuck in a spider's web, the spider doesn't release it. Lorelia is much the same. It's nature."
:So you realize what she's doing, and and you still support her?
:"I do. I support the survival of lake Halcyon. So much life depends on it. Sacrifices must be made, and to prevent this from becoming a dry, dead desert, it's worth it. You must decide as well. If you wish to aid us, Lorelia would speak with you."

The Choice
You will now have a choice, to side with Herald Kixathi and the Seekers or the nereid and her Rain Disciples. The following is what their respective sides have done:

Herald Kixathi has taken followers of the nereid hostage and has tortured them.  Lake Halcyon will eventually turn to desert, but the nereid wants to protect the waters.  This would allow future generations to survive and prosper.  Herald Kixathi says that this is a violation of the natural order, and to impede the natural evolution of the landscape is unacceptable.

However, the nereid has used her followers to her own ends, and has robbed them of their memories.  Additionally, she shows little regard for the lives and safety of her followers, which suggests the nereid does not care about any mortals.

You can choose to aid Kixathi by agreeing to help her during her confrontation with you.
 

Choosing to aid Kixathi again leads to Sunken Knowledge. 

However, if you decide to aid the Rain Disciples, you can find Apostle Marian nearby who will start The Water Stone.
 

Notes
  

Quest Stages