Surtr is a Boss in Assassin Creed Valhalla which can be found during [The Recue] quest. In this quest, you are in search of Baldr who is the son of your in-game character.
While finding your son you will reach a place named [Eitri]. This place is surrounded by lava around and inside same as well.
After you interact with this place inside you will see your son hanging.
Your wife will rush towards him but you will encounter a woman and then Baldr. You and Baldr talk about your fight before and ask for the final battle. After some talk, your wife will punch the lady shortly after that enemies will appear from the ground and take your wife away and fight with Baldr begin. This is the first time you will encounter him for the first time.
Baldr is a big giant boss with a long sword in his right hand which lightens up while he attacks. He has multiple attacks like left-leg smash, left-hand smash, and sideswipe sword.
How to defeat
When he smashes his left leg on the ground vertical lava lines will appear from the ground you can save yourself by creating some distance. When he smashes his left hand on the ground at the place you are standing a lava circle will appear. You can dodge this attack by rolling left or right.
He will also swipe his sword from right to left or left to right. You can roll on the opposite side of the attack to dodge. In the end, he will also use his hands to throw fireballs which will follow you on the ground.
After defeating him a cinematic will run in which your wife is fighting with the appearing enemies. Your wife will scream to look behind and you will be attacked by Baldr and he will pick you up from his hand and throw you on the wall. You will see your wife fighting with them and they will kill her by slitting her neck.
After that Baldr will grab you from your leg after that you will wake up in a lot of corpses that will be the end of this fight and quest.