Blood on the Battlefield - Kaer Morhen
This quest begins immediately following The Battle of Kaer Morhen Quest. During this quest you will have to respond a certain way to the conversation to change the game's ending. I can't tell you what the changes will be without major spoilers, however I will label the dialogue options as "positive effect" and "negative effect" on the game's ending.
When you begin this quest you'll be in front of Vesemir's pyre, speak to anyone that you want to here then return to Kaer Morhen for another scene. A few days will pass and Ciri will come to Geralt for advice and this is where your choice matters.
"Think I know what might lift your spirits" - this has a positive effect on the game's ending
"Relax. You don't have to be so good at everything." - this has a negative effect on the game's ending
Geralt and Ciri will have a snowball fight if you answer with #1 whereas #2 they'll drink their sorrows away. Your next two choices will come a little later when Ciri asks you whether or not you should first go to Velen or go see the Emperor. This is one of the most major choices as it influences Ciri's ultimate decision.
Choose to see the Emperor first: Ciri will decide to become an Empress
Skip right to Velen: Ciri will decide to become a Witcher
If you decide to go see the Emperor then you'll have another major dialogue choice to make there. When he offers you a reward in front of Ciri - refuse.
Accept Gold - negative effect
Refuse Gold - positive effect
Ciri will think money motivated you if you accept the reward, if you refuse it then she'll know that your love for her is what motivated you to find her. There will be much more dialogue and some scenes before the end of this quest. The next quest, Bald Mountain, will automatically be added to your quest journal.