Brothers in Arms: Velen is one of the recruitment quests before The Battle of Kaer Morhen. In Velen, there is really only one person who could make a difference in a fight against the Wild Hunt: the sorceress Keira Metz.
Whether she comes depends on how Geralt handled her storyline, especially the ending of For the Advancement of Learning. Keira can seem vain and self-absorbed, but deep down she cares about the fate of the world. If Geralt pushed her the right way and sent her to Kaer Morhen, she promises to stand and fight. If things went differently, she is too wrapped up in her own troubles, or worse, and Geralt has to go on without her.
Her presence at Kaer Morhen matters more than it might seem, because Keira can make the difference between life and death for one of the witchers during the battle.
