Tedd Deireadh The Final Age - Skellige
This is the final quest in Witcher 3 and it begins immediately after you complete On Thin Ice and defeat Eredin. For the first part of this quest you'll need to follow Yennefer up the mountain and to a group of two horses. If you get hit with fireballs along the way use whatever food or potions you have available.
On the horses you will have to dodge Ice Giants, Hounds of the Wild Hunt amongst other enemies. They will be portaling in all along the route you're going to be riding along with the horses. I highly recommend you never use sprint on your horse while following Yennefer - she takes a really wacky route through this entire area and your sprint is much slower than hers.
When you reach the start of the mountain you'll have to ditch the horses and follow Yennefer up the mountain on foot. She will conjure up a protective barrier to keep you and her safe, if you step outside of this barrier you'll take damage from the frost.
On your journey up the mountain side you'll have to fight off some Hounds of the Wild Hunt, let the hound come back to you so you can stay inside of the barrier while you fight it. Eventually you'll reach a barrier that Avallac'h created nearby a tower. Yennefer will open up a tear for Geralt to pass through but she must stay behind.
Follow the linear path up to the tower with Geralt to have one of your final cutscenes of the game. After this quest is complete you'll receive the quest Something Ends, Something Begins which is the true final quest of the game. Your quest will be different depending on what side quests you did and what dialogue options you chose when prompted.