You get the quest "Laid to Rest" in Morthal. Talk to the NPCs there. They mention a burnt down house. Ask if there is a story behind that burned down house. It's bad luck to talk about that place, they tell you. Hroggar's house? It burned down not too long ago. Jarl's been looking for someone who ain't superstitious though. Be lookin' for a fool if you ask me. You should then receive the quest telling you to go speak to the Jarl of Morthal. If you do not get it by talking to guards, innkeepers or NPCs, try heading straight to the Jarl and speak with him.
If you talk to Hroggar and tell him you heard his house burned down, he
does not seem to care much. He is in a new and better house right now,
and even though it was a shame to lose his wife and child, now Alva
takes care of him and he takes care of her. You can pickpocket Hroggar
if you want, to get the key to Alva's house.
If you speak to Alva
(at night) and tell her about the rumors that Hroggar burned down the
house for her, she tells you how lucky she is to have a man so devoted
to her. She does not seem to care much about the rumors either.
Open your quest Journal:
Laid to Rest
A mysterious fire killed Hroggar's wife and child. Hroggar took Alva as a lover immediately after the fire, and many in the town think he did it. The Jarl might pay me to investigate it.
Your first objective in this mission is to talk to the Jarl. Enter Highmoon Hall in Morthal and speak with Jarl Idgrod Ravencrone (or Sorli the Builder). She tells you that Tamriel is full of wisdom and magic if one is willing to look for it. Few have their eyes open.
From the next dialog choice, choose: "I hear you want someone to look into that house fire." She tells you Hroggar lost his wife and daughter in the blaze. Her people believe it to be cursed now. Who is she to gainsay them? Ask her what Hroggar says happened. She tells you that Hroggar blames his wife for spilling bear fat in the fire but many folks believe he set the fire himself. You ask here why would he do something like that to his own family. Lust can make a man do the unthinkable, the Jarl replies. The ashes were sill warm when he pledged himself to Alva.
When you ask the Jarl why they have not made an arrest, she says they cannot arrest him based on rumor and gossip. But since you are a stranger, you might be able to find the truth for them. She will reward you if you prove him guilty (or innocent).
Your objective now is to investigate the burned house. Follow the quest marker to reach it. You will run into a ghost of a girl named Helgi. Speak to Helgi's ghost. Ask her who she is, and if she knows what happened to her house. She is lonely and wants you to play with her. Ask her if she will tell you who set the house on fire if you play with her. She agrees to tell you if you play hide and seek with her. She refers to the "other one" that comes out only at night time. So your objective now is to find Helgi after dark to play hide and seek with her.
At Nightime, head west in Morthal up the path to the right of the house. You will run into Laelette the Vampire standing near a coffin. Kill her. Then activate the Child's Coffin. Helgi is happy that you found here. She tells you about Laelette's involvement in this.
Speak to Thonnir, Laelette's husband, and ask him what he can tell you about his wife. He does not believe that her and Alva are vampires. Your goal now is to investigate Alva's house.
During the night, you can also find Alva at Moorside Inn for additional dialog. If you ask Jonna about Laelette, you learn she ran away when the war started, leaving Thonnir devastated. People thought she went to join the stormcloaks.
Go to Alva's house. It has an adept lock on it, so shouldn't be that hard to pick the lock. Or use the key if you pickpocketed it from either Hroggar or Alva earlier.
If you entered during the day, Alva is sleeping in the coffin downstairs. During the night, Alva is out but Hroggar could see you and attack you. So it is best to come at night. Head down to the Cellar downstairs. Alva is sleeping in her coffin but hears you and wakes up, then attacks you. Now, look inside the coffin to find Alva's Journal. Read it. It mentions a vampire lord called Morvath who instructed Alva to seduce Hroggar and use him as a protector while she takes over town making all the guards her slaves. But Hroggar's family is in the way and she had asked Lalette to kill them and make it look like an accident.
Your objective now is to show Alva's journal to the Jarl. Return to the Jarl and tell her you have Alva's journal. After hearing about the situation, she wants you to go to Movarth's lair and kill the master vampire. She will gather people outside to help you, NPCs such as Thonnir and other Morthal Citizens. Lead the way to Movarth's Lair. Thonnir and the others get there too. But the other citizens are afraid to go in. You can ask Thonnir, who is brave, to come with you, or have him wait outsise while you avenge his wife's, Laelette, death.
Enter the caverns and make your way to the main chamber where Movarth Piquine is in. He is sitting on a throne. There will be some more vampires with him such as a Volkihar Vampire.
Kill Morvath, then return to the Jarl and inform her that the Master Vampire Morvath is dead. This concludes the quest "Laid to Rest".
