The Harbinger of Us All, the fourth quest in the Blood of Kings quest line, is automatically started once both of its prerequisite quests -Faces and Names and The Bloodsmith– are completed. Not having heard from Larsdon & associates in a while, your first objective is to wait for a message from him.
A courier should deliver the ‘Note to Dragonborn’ along with a key. In the note Larsdon expresses fear for his life and tells the player to meet him in an abandoned shack (Greenwood Shack) in the mountains southeast of Falkreath.
Find the secret entrance to the cellar under the hay there, and enter it to reveal Valgus, Falatild, and Karras- whom you may remember as the person Larsdon was speaking with at the beginning of From The Blood Of Kings. They confirm that Larsdon is indeed missing. Falatild just about has a conniption fit, and tells the group that there is no little girl, that, unbeknownst to even Larsdon himself, he is the bastard of Ysgramor’s blood.
After finding out that curveball, it’s time to report all the information you’ve found. Go talk to Karras who will ask you to meet him outside before revealing that he knew about Gren and his efforts to claim the throne and that he must not come to power. He then directs the Dragonborn (and Valgus) to go to Gren’s camp and speak to him and his subordinates in order to try and locate Larsdon.
After meeting Gren and delivering your ultimatum, talk to the people around the camp. Eventually they will direct you to find the mage with the wolf ring whom we sought out in the quest Faces and Names. Because the people there speak in riddles, it is unclear exactly where they are talking about, but they do mention ‘a mill of heart’s wood’ (heartwood mill) located to the south. There you will find Anum-La, who will join your quest and direct you west to Angarvunde. Head to the catacombs where you will find rubble that has been cleared recently. Continue on into Angarvunde depths where you will find Larsdon. You will also find another of Eldritch’s journals. Once outside, Anum-La will depart, saying that she has business in Riften.
In any case, head back to Greenwood shack where Larsdon will rebuke Falatild for concealing his heritage from him. He will then proceed to complete the ritual, with the screen blacking out just as the axe is driven into Larsdon’s body.
When the Player comes to, Karras will engage the Player in conversation, revealing that although he suffered grave injuries, Larsdon managed to survive. He will proceed to update the player about the goings-on of the others, as well as to reward the Dragonborn for their help, thus completing the quest.