You can easily start this quest while doing the Main Quest: Elder Knowledge, as it takes us right to where we need to be to start this quest. While doing that main Quest, you’ll reach the Dwarven dungeon of Alftand. After the section where you kill Sulla Trebatius and Umana, you will go down some stairs and enter Blackreach.
Once inside this beautiful cave, go directly in front of you and enter Sinderion’s Field Laboratory. In here, grab the Crimson Nirnroot on the right side and read Sinderion’s Journal to start the quest. READING that journal is what will start this quest.
Now, in the cave outside the Laboratory, you need to find 30+ Crimson Nirnroots. Given how utterly HUGE and wide-open this area is, I can’t exactly provide you a map of where they all are. There’s a ton of them, that’s for sure! What you need to do is just go out there (well, out there INSIDE the cave) and walk around while looking for them. They are exactly the same as the normal Nirnroots. They shine and they make a soft ringing sound.
What you need to do is scavange the place and then leave Blackreach through one of the lifts, then wait around 48/72 hours and return so the Nirnroots will be forced to re-spawn. Keep on grabbing the Nirnroots until you have 30. Once you have 30, you will be told to bring them to Avrusa Sarethi. Follow the marker to find a way out of Blackreach (it’s so much easier with a marker!) and you’ll see that Avrusa is located east of Ivarstead. Indeed, she actually has her own farm called Sarethi Farm. Fast travel as close as you can to it and head on over there.
Wait until morning and Avrusa will be out tending her crops. As you can tell, she is quite the nirnroot farmer. She was Sinderion’s student after all. Chat with her and tell her of Sinderion’s fate and then give her the 30 Crimson Nirnroots. She will thank you and end up giving you a tome of Sinderion’s, but that’s not all! You will also receive a magical permanent effect:
o SINDERION’S SERENDIPITY: There is a 25% chance of creating a duplicate potion when using your alchemy skill.
And with that, the quest will be over. Congrats on finishing a very rough quest!