Brothers in Arms: Velen
This quest begins automatically upon completing the Ugly Baby Story Quest. For most people this quest will automatically complete as there are only two people that you need to speak to for this quest and one of the is completely optional/requires a Witcher 2 Save File.
Keira Metz is the other person and assuming you've completed An Invitation from Keira Metz, A Towerful of Mice, A Favor for a Friend and For the Advancement of Learning - she will either be dead or have agreed to help you already with your battle at Kaer Morhen.
Note: If you completed Letho's Ghosts of the Past Quest then he will automatically join you at Kaer Morhen too