Brothers in Arms: Skellige is one of the four recruitment quests that Geralt can take on before The Battle of Kaer Morhen. Nobody fights harder than the warriors of Skellige, so Geralt travels to the isles looking for anyone willing to stand with him against the Wild Hunt.
The timing is not ideal. The whole archipelago is caught up in the question of who will succeed King Bran, and Geralt has his doubts about finding help at all. Who comes depends on what he did earlier. If he helped Hjalmar an Craite in The Lord of Undvik, Hjalmar and his men will come, and depending on how other Skellige quests went, other friends may follow.
It is a quest that rewards earlier choices: the more Geralt has helped the people of Skellige, the more of them turn up at Kaer Morhen when it matters.
