The Blood and Bones quest is automatically given after its prerequisite quest, The Harbinger of Us All, is completed. The Dragonborn’s first objective is to find out what happened to the others- Valgus, Isobel and Larsdon. Talking to them will produce a variety of interesting banter, but the person we’re really after is Falatild. So go to Rorikstead and head on over to the Frostfruit Inn.
You can make small talk if you wish, but once you ask what she is doing in Rorikstead, the two of you will have some unexpected company- Gren and his men will come in and order the bard Yarborough, to start playing. When Falatild will try to leave, Gren recognizes her, as well as the player. Without further ado, he orders the Player and Falatild to kill Eldritch, the war-teacher, whom he now considers an enemy. Apparently, the mage has holed himself up in Bleakcoast Cave to the northeast, withstanding the scores of men Gren sent to try to kill him.
Travel to the cave and make your way through it to enter the attached crypt- Bleakcoast Ruins. There you will find hordes of dead Blood of Skyrim grunts, as well as hordes of undead (and also a few werewolves). Eldritch will appear to you several times while traversing the dungeon, hurling a bunch of cryptic lines at the Player which probably don’t make much sense to you right now.
Eventually you will come to the end of the ruins, where you will find Eldritch, in his beast form once you slay him, check his body and obtain his wolf ring. This will summon a spirit wolf who will reveal to you that the ritual is all a lie- when the king defeats the beast, the king himself becomes a beast. After a conversation with the wolf and Falatild, the quest will be complete.