The Dragonborn must have the Glenmoril Witch head(s), (see Blood's Honor), in the inventory before accepting a radiant quest, or Purity will not show up. Either the first or second time Farkas or Vilkas is talked to, they will ask for help on a quest of a personal matter; to go with them back to Ysgramor's Tomb and perform the same ritual that was done with Kodlak.
If the witch heads are stored somewhere and then retrieved after completing a few radiant quests, Purity will not show up immediately when talking to Farkas or Vilkas. The player must first complete a radiant quest from each of them, with the witch head(s) in the inventory to make Purity available.
When asking from a job from either Vilkas or Farkas, if they have the option "Is there any work to be done?", they will offer this quest.
To avoid bugs, first speak to Vilkas. He gives a random quest, then he gives his Purity quest, (remember to have the head(s) in the inventory). Decline his Purity quest. If Farkas doesn't give any quest after that, take an Aela's random quest and complete it. This will unblock Farkas random quest. Do it and then complete his Purity quest. When that's done, Vilkas will complete his Purity quest without a problem.
PC 360 PS3 After declining Vilkas' Purity quest neither Farkas nor Aela have the option for any sidequests in their dialogue - only the option to 'Follow Me'. When attempting Vilkas' Purity quest he leaves the player as soon as they step outside of Jorrvaskr and the quest is moved to the 'Completed' list. Speaking to Vilkas again only offers the 'Dragon Seekers' quest. When attempting to recruit a follower NPCs will note that the player already has someone with them when they do not. This bug is due to the Purity quest.
After completing Purity with either Farkas or Vilkas, if one immediately talks to the other one, sometimes the quest will show up in the completed quest section, and the altar in Ysgramor's Toumb will not be able to be activated. This can be solved by loading a save state from before the first Purity mission was accepted, re-completing it, then doing another mission. Any mission should do, an Animal Extermination mission from Aela does the job nicely. After doing another mission, talking to the second of the two brothers starts the mission the way it's supposed to and it can be completed.
There appears to be a bug if Vilkas's Purity is accepted quest before Farkas. The quest status will switch to the Completed even though it hasn't been completed. Also Vilkas will not follow the Dragonborn to Ysgramor's Tomb. To resolve this, accept and finish Farkas's Purity quest first and be sure that the quest giver is following the whole way through.
A quick fix is to exit the conversation when the speach choices are "get up", "are you better" or "I have killed it", then spam use on the altar.
Sometimes after activating the Flame of the Harbinger one will still have the same number of Witch Heads as before. To fix this just drop and pick them up at the altar.
Travel to the tomb by any means and stand next to the fire
Tell him it is time to part ways
Immediately speak with him again and accept quest
Click fire and kill wolf spirit
PC If all above fails, then console command:
setstage cr13 10 - this restarts the quest
setstage cr13 20 - if there is a witch head already in inventory
setstage cr13 100 - finishes quest
360 If one does not get cured immediately after "Glory of the Dead," the Flame of the Harbinger cannot be used unless a side contract from BOTH Farkas and Vilkas has been completed. Do one contract from each of them, then the second time one asks for work they will start the "Purity" quest.
360 If Vilkas has already been talked to about being cured and he won't leave Jorrvaskr then try this. Walk back in and talk to Vilkas, Purity quest should be in the completed bin, he should offer work. If he does accept it and finish the mission he gives. When the misson is completed, Farkas and Aela should now offer work. Talk to Farkas and he will ask to be cured, take Farkas to the site and cure him. The player should now be able to have a Follower. Once Farkas is cured, Vilkas should do Purity with him without him bugging out.
360 If Farkas is married to the Dragonborn, he no longer gives quests, even though the quest indicator says that one can get work from the Companions.
360 Quick Fix: If Jorrvaskr cannot be exited, disable autosaves. A black screen appears. Activate the map and move until hearing a click. While the cursor is hovering over a destination tap A twice to fast-travel. If the result is a black screen for more than two minutes the fast-travel has failed.
PC This fix is for the glitch involving Vilkas where he is locked to the player as a follower and cannot be gotten rid of by telling him to "part ways." This is most likely after taking on the "Dragon Seeker" quest with him and cannot complete it because Bonestrewn Crest has already been cleared. It is not confirmed to work with the glitch where Vilkas chases the Dragonborn around as a broken guard trying to collect a bounty. It is also not confirmed if it works in reverse with Farkas as the glitched follower. Anyway, to cure the glitch:
Create a manual save file before proceeding in case something fails.
This isn't necessary but be outside Jorrvaskr to make sure its working right.
Vilkas must be locked to the player, blocking work from the other Companions.
Enter the command "resetquest cr14" into the console, this resets the "Dragonseeker Quest" with him; Vilkas will leave and go inside Jorvaskr and no longer be a Follower, but it still will not allow a new follower.
Enter the command "setstage cr13 200" to complete the "Purity" quest with Vilkas that is bugged; it is actually active in the background but does not appear in the COMPLETE or INCOMPLETE quest lists.
One will now be allowed to seek jobs from other Companion members
IMMEDIATELY go find Farkas inside Jorrvaskr.
Ask him for work, he may do one of two things:
Whine about never seeing a dragon and give his "Dragon Seeker" quest.
If "Dragon Seeker" enables, fast travel to Northwind Summit.
IMMEDIATELY talk to him and he should start talking about something that is bothering him, ("Purity" quest), even if there is a dragon attacking.
If this doesn't happen reload the save file and try again.
The "Purity" quest will now enable.
Immediately say something is bothering him and give his "Purity" quest.
If "Purity" enables, then proceed as normal.
Once his "Purity" quest enables he will REMOVE the other Follower (Vilkas) and become the new Follower.
Go to the Tomb of Ysgramor and proceed to the cleansing fire.
Place a witch's head into the fire and fight Farkas' wolf spirit and kill it.
Talk to Farkas and ask him if "he is alright" and complete the quest.
At this point purify one's self if desired.
Go to another companion and "follow me" should no longer be grayed out
CREATE A MANUAL SAVE FILE
Attempt the "Purity" quest with Vilkas at one's own risk now