When you reach your destination, which is a Nilfgaardian camp set on Marlin Coast, Avallac'h will be explaining his plan to everyone, however Ciri will not be allowed to take part in the battle. After everything is explained he will tell Geralt to prepare his potions and everything before they start - good idea so do just that while you have the chance. If you need repairs you can get them from a Nilfgaardian blacksmith.
Note: If you romanced either Triss or Yennefer, Geralt will have one more conversation with the one he chose before you head out.
The Battle Begins
Once you are ready, talk to Avallac'h to let the battle begin. Avallac'h and Ciri will use the Sunstone, calling the Naglfar to Undvik. However, Caranthir will freeze everyone and Ciri will head out to save them. You will now be controlling Ciri, who will unleash her Elder Blood power. She will activate a magic aura and begin working through several groups of Wild Hunt enemies, where she can literally one-shot all of them. If you use her Blink ability, she will wipe out up to 5 enemies within range. Once you reach Caranthir, you will have a short battle. After taking him down by a third of his HP, a cutscene will trigger and Ciri will break the orb on his staff. However, the explosion will incapacitate her, but not before she manages to escape. Fortunately, Geralt will thaw out just in time to continue the fight.
The Last General
Caranthir will pelt Geralt with magic projectiles and teleport away from him at regular intervals. He will also call Ice Elementals to join the fight. If you start getting overwhelmed, keep your distance from him and position yourself so the Elementals block his line of sight, as his attacks will harm them too, if hit. Quen and Yrden combined with the Magic Trap ability can be effective for blocking his attacks. Do not worry about saving potions and bombs for fighting Eredin, as you can meditate without any time lapse immediately after this fight. Once Caranthir is near 5% HP, a cutscene will trigger. Geralt will pierce his sword completely through Caranthir. However, Caranthir will then teleport them both deep underwater in a last attempt to kill Geralt. The cutscene ends and you will now have to the surface as quickly as possible. This is fairly easy to do, but Killer Whale can also help ensure you have enough time.
Now meditate if you need to and head out. Fight your way toward Naglfar, along the way you may encounter Hjalmar, however this is not the time to talk so just keep pushing forward. When you reach the Naglfar, another cutscene will trigger where Eredin will be fighting Crach an Craite. Crach, who is badly injured, manages to grab his axe and stand up again to continue fighting. Eredin asks if he has any last words, and Crach will tell him to shut his trap and fight. Crach lunges at him one last time but Eredin cuts him down with one final blow. Geralt will appear and Eredin says he's got something to tell him, but will wait to do so until after they fight.
The King of The Wild Hunt
Eredin has no one to lean on anymore, however don't count him out just yet and use everything you have to put him down. When the fight starts, he will taunt Geralt to show him his famous pirouette. Like other Aen Elle, he will quickly teleport from one spot to another, similar to the previous fight against Caranthir. From a distance, Eredin will attack with magic, but will use his sword while in melee range. He is relatively easy to stagger and using using the Quen Exploding Shield ability can help with this. Using the Whirl combat ability can be effective if used methodically and activated at the right time, but Eredin is very good at parrying and will block every strike if you use it at the wrong time. After landing a few blows, he will throw Geralt through a portal and they will continue their fight on an icey plateau. Once Eredin is almost beaten, he will open a portal back to the Naglfar. Large magic projectiles will begin falling from the sky, so make your way to the portal as quickly as possible, follow him through and finish the fight. Once he is down to 0 HP, a cutscene will trigger where their swords clash and the force of Eredin's strike will knock Geralt back. Geralt will then use the momentum to perform a pirouette (the irony is strong) and direct his sword to Eredin's face, plucking his eye out. After one last exchange, Geralt will slash his throat and Eredin falls to the ground. Geralt asks what he wanted to tell him, and Eredin says that they were both fooled by Avallac'h, who supposedly set them against one another so he could make off with Ciri. Eredin then dies and hounds of the Wild Hunt will begin closing in. Yennefer will appear, teleport them away, and the quest will end.
- Every story, even the longest epic, must end, and so this one too now nears its conclusion. Geralt and his friends had acquired the Sunstone and secured the help of Fringilla Vigo. In other words, the long and dangerous process of preparing was over at last and everything was in place for the final battle against the Wild Hunt. All that remained was to summon the Naglfar to the shores of Undvik - and straight into their trap.
- Every plan looks best on paper. It was no different this time - though every last detail had been seen to, our heroes were still in for many an unexpected danger and difficult moment. Their division of elite Nilfgaardian soldiers was frozen in ice before it could board the Naglfar. Ciri, who was supposed to stay out of the fighting, was forced into the fray to save Geralt. Meanwhile glory-seeking, fearless (or reckless, some might say) Skelligers rushed into the bay to fight both the riders of the Hunt and the Imperial Fleet at the same time.
- Despite all these complications, in the end the allies defeated the Wild Hunt and Geralt slew its king with his own hands. It was too early to rejoice, however - right before his death Eredin revealed that Avallac'h had betrayed the witcher and had his own nefarious plans for Ciri. Geralt did not know whether to trust his enemy's words - but circumstances did not allow for long and careful deliberation.
- Join Avallac'h and the sorceresses on Undvik.
- Talk to Avallac'h when you're ready to fight.
- Defeat Caranthir.
- Kill Caranthir.
- Swim to the surface.
- Board the Naglfar.
- Kill Eredin.
- Follow Eredin into the portal.
- Find Avallac'h.
- The quest Reason of State must be completed before speaking with Avallac'h, otherwise it'll fail.
- The Marlin Coast settlement is now a hive of activity with the soldiers encamped there. As a result, a few changes have occurred: one of the buildings is now fully stocked with alchemy supplies and some weapons while another has been converted into a barracks, a quartermaster is now available for crafting weapons or buying and selling goods, and the immediate region has been cleared of randomly spawning monsters.
- After the briefing, Ciri goes off for a walk. Before commencing the battle, if Geralt follows the path into the hills that starts next to where the Quartermaster is based, he can find her a little ways from the camp, practicing her magic by levitating rocks and where one can converse with her for a bit if they wish.
- If Geralt is romancing Triss or Yennefer, one of them will come to visit Geralt in the tent after Avallac'h tells Geralt to meet him at the dock.
- Speaking to Avallac'h at Marlin Coast and saying Geralt is ready to fight is a point of no return.
- There's a small chance for Caranthir to freeze up for the rest of the boss battle after the player character has been swapped from Ciri to Geralt. This glitch seems to occur at random and can be undone by reloading your previous checkpoint, though if he can be damaged, the game will progress after his health is lowered to zero.