Complete secondary quests to gather allies from Velen to assist Geralt, Yennefer and the witchers of Kaer Morhen in a battle against The Wild Hunt.
The Witcher 3: Brothers in Arms: Velen
Between Va Fail, Elaine and The Isle of Mists it is in your best interests to gather allies. If you've been completing secondary quests as they arise this is no difficult task, but if not, you have quite the slog ahead of you.
There are three Brothers in Arms quests, one for each major region. Here are our recommendations for getting the job done in Velen:
A former advisor to King Foltest, the sorceress Keira Metz is hiding out in Velen. Help her out with her projects to secure her assistance in the great battle ahead - and optionally get your bow-chika-wow-wow on.
It's not listed in the quest guide, but if Letho is still alive in your game, you can bring him to Kaer Morhen, too.
With these tasks complete you can attend to Brothers in Arms: Novigrad and Brothers in Arms: Skellige, or if you have completed all three regions, go on to The Isle of Mists.