I've agreed to help the Hermaeus Mora cult of Primeval Seekers to prevent the nereid of Halcyon Lake from using forbidden secrets to subvert the laws of nature.
Head over to the shrine itself. You will find the Voice of Hermaeus Mora hovering over it. Talk with it to complete things here.
Voice of Hermaeus Mora: "So you are the one the tides of fate carried here to stop this transgression?
"I have watched this pathetic spirits attempt to avoid her fate with some amusement ... but I knew it would eventually have come to an end."
Vestige: "Why didn't you just stop her yourself?"
Voice of Hermaeus Mora: "Where would be the fun in that?
"The individual yarns of fate do not concern me. It is the tapestry they weave I watch and record. All threads, even yours, only make up a small part of the whole."
Vestige: "So you know my fate?"
Voice of Hermaeus Mora: "I do. I can see your future ... and your past. The yarn of your life is interesting, perhaps even amusing, one might say.
"It's spun from a strong thread, to be sure, but all threads will eventually run out."
Vestige: "Can you tell me anything about my future."
Voice of Hermaeus Mora: "No."
Vestige: "Why did you summon your followers here?
Voice of Hermaeus Mora: "I did no such thing. Like moths to flame, my followers are simply drawn to dark mysteries, and this one was simply irresistable for them.
"It's a wonder more of them didn't die."
Vestige: "You don't care whether your followers live or die?"
Voice of Hermaeus Mora: "No. Why would I?"
Vestige: "If there's nothing more, then I will go."
Voice of Hermaeus Mora: " Yes. That is what you will do.
"First though, I have something for you. A gift, you might say."
Vestige: "Is this a gift something I actually want?"
Voice of Hermaeus Mora: "Whether you want it or not is irrelevant. I like to reward those who amuse me, and this is your reward."