Bloodmoon:Rite of the Wolf Giver

Quick Walkthrough
# Hear about the rumor of a large black raven at the Altar of Thrond.
# Talk to the raven, which is actually a witch of Glenmoril Wyrd, and accept its offering to cure your lycanthropy.
# Talk to the witches in the Gloomy Cave and learn the steps of the Rite of the Wolf Giver.
# Get some petals of a wolfsbane flower from Hvitkald Peak.
# Get some ripened belladonna berries near a burial barrow or crypt, or from a spriggan.
# Go back to Gloomy Cave and give the witches their required components.
# Meet Ettiene back at the altar to complete the next steps.
# Sacrifice the innocent Nord on the altar and give her heart to Ettiene.
# Place the Heart of the Wolf back in the body of the innocent and talk again to Ettiene.
# Kill the werewolf you just created and be cured of and immunized against lycanthropy.

Detailed Walkthrough
Find the Rumor
Once you've become a werewolf during the Main Quest, it is possible to discover a cure which will cure you permanently. The first step in this quest is to find the note to get you started. You can find a note entitled Odd Rumor lying beside Basks-In-The-Sun down at Fort Frostmoth docks. The note mentions that a large black raven has been seen at the Altar of Thrond and may be a sign that witches have come to the island.

Altar of Thrond
The altar is found outside roughly in the center of Solstheim, north of the start of the Harstaad River and Isinfier Plains. When you read the note the location should be marked on your world map. The altar is a small circle of stones and you should spot a large Raven standing in the entrance of it. The raven will speak to you as you approach it and offer you the choice to either keep or cure your lycanthropy. You can refuse her and come back later, or kill Ettiene of Glenmoril Wyrd, the woman into which the raven transforms, for a note that guides you to an icicle in the Gloomy Cave.

Road to the Cure
Once you choose to cure yourself of lycanthropy, you are teleported into the Gloomy Cave with the Glenmoril Witches. The witches will tell you what to start with for the Rite of the Wolf Giver as you approach them. Fallaise will tell you your first task, that of finding some petals of the rare wolfsbane flower. The flower grows in only one place on the island, the top of Hvitkald Peak. The peak is to the west of the cave and altar and should be marked on your map (though the flower location is just west of that mark). You may need to use the levitation potion the witches gave you in order to reach the peak as the slope is quite steep.
A sample of Wolfsbane Petals can also be found in Lassnr's House in the Skaal Village.

Belladonna Berries
Return the flower to Fallaise who will then tell you to speak to Isobel. Isobel will simply tell you to acquire a single bunch of ripened belladonna berries. The berries are relatively common and can typically be found on spriggans or next to barrows' entrance. Return the berry to Isobel who will direct you to Ettiene. Ettiene will tell you to meet her at the Altar of Thrond for the next steps.

Innocent Sacrifice
Back at the altar, Ettiene will tell you to kill the innocent Nord on the altar and take her heart. Since there seems to be no way around this, do what she asks and give the heart to Ettiene. Ettiene will perform a short ceremony on the heart and give you back the Heart of the Wolf which you are to place back into the corpse of the innocent Nord, so don't dispose of the corpse when you first get the heart. Once the heart is placed back into the corpse you'll receive a journal entry.

End of the Curse
thumb|right|Defeat the werewolf
Speak to Ettiene at the altar again and she'll tell you that you have to kill the new werewolf you just created by giving the innocent Nord the Heart of the Wolf, and your lycanthropy with it. The corpse on the altar will turn into a werewolf and attack you, so kill it. Once you kill it and speak to Ettiene again she will tell you that you've been cured permanently. You will not be able to become a werewolf nor get infected by the disease again after this. Be warned, if you kill the new werewolf when you are a werewolf, then you will never transform back into a human, even in the day! That can be resolved, however, by typing   into the console if you are playing on PC.

Alternate Method
If you wish, you can simply kill the witches. They each carry a Note from a Glenmoril Witch which advises you that an emergency scroll to cure lycanthropy has been hidden in a hollowed out icicle hanging from the roof of the Gloomy Cave. The icicle is disabled once you start Hircine's quests, but is enabled again after completing them. Levitate up to this scroll (sc_lycanthropycure) and use it in your inventory to cure yourself.

It is possible to spare the witches who are willing to help you by simply stealing the letter, reading it, and then proceeding as written above.

Return to the Skaal
If you have not yet begun Hircine's quests, you can return to Korst Wind-Eye at the Skaal Village and he will allow you to side with the Skaal and continue with the main quest. Note that you will need to cure yourself before your second werewolf transformation if you still wish to side with the Skaal. 

If you have already completed the main quest, you can return to Skaal Village and speak to Korst to be named Blodskaal. The Skaal will be happy to welcome you back, and you will be given the Bloodskaal's House for use as a player home.

Notes
Ettiene will offer the use of the unique Wolfgiver Silver Dagger on the altar to kill the werewolf. However, as she says, any weapon will do.
If you lack the means to levitate you can also jump off the cauldron, but this may take a few tries as you get a split second to click the icicle. With a sufficiently high acrobatics level (around 70) you can jump from the floor high enough to reach it.
If you decide to kill the witches, their bodies will become permanent corpses, making the Gloomy Cave an excellent storage cache.
If you wish to acquire the Note from a Glenmoril Witch, the unique dagger mentioned above and still cure your lycanthropy by using the scroll and not by killing the innocent, you must steal the Note from a Glenmoril Witch from either Fallaise, Isobel or both any time prior to leaving the cave to meet Etienne at the altar as Etienne herself does not carry a copy of the note and the two sisters will disappear from the cave once you leave to go meet Etienne. Upon meeting Etienne at the altar, you may then take the dagger and leave, thereby condemning the innocent woman and Etienne to forever remain at the altar.

Bugs
 
Unequipping the Mantle prior to this quest can fix this.
 
 

Quest Stages