The Beast Who Cannot Be Slain continues where its prerequisite quest, Blood and Bones, left off. This quest involves (what else) the slaying of THE BEAST! So let’s get on with it!
Hurry back to Greenwood Shack where you will find it is too late- Karras tells of Larsdon’s transformation, how he savagely attacked him, and how Karras would have died had his men not pulled him out of the cellar. Karras pleads with Falatild to kill Larsdon, however the housecarl expresses great reluctance. Will she do it? Who knows…
In any case, enter the cellar and go down to the forge area to find Larsdon- who is now in the form of a dragon. He flies off just as you and Falatild enter the cavern. Falatild will then outright refuse to kill him. The spirit wolf appears and presents you with another option; you can use your blood, that of a dragon’s- to save him. Just use the spell from the transfusion tome you picked up earlier. Go outside and talk to Karras who will tell you that Larsdon has flown north to Rorikstead. He also mentions that his guards are at the player’s disposable as well, providing you go inside the cellar and talk to them.
Whatever your choice, head to Rorikstead- but make sure you’re well prepared first. You will be facing the whole of the Blood of Skyrim as well as Larsdon the Dragon attacking you all at once.
Your next objective is to defeat Larsdon (aka The Beast), either with or without the use of the transfusion spell. In either case, defeating him completes the quest.