If you choose to have mercy, you can spare King Charles’ life in the new AC DLC. This guide on How To Spare The Kings Life In Siege Of Paris For AC Valhalla will tell you exactly how you save the king so that you can trap him and not have to kill him, resulting in a different ending to the DLC’s main story.
I am not sure if this option is available in both battles. When you speak with the queen, before the fight with King Charles, she will give you the choice to either kill the king or spare the king. If you choose to kill the king, the option to save his life in the final battle is not available. So, if you want to spare the king at the end of the story, you must respond with that choice when speaking with the queen.
How To Spare The Kings Life In Siege Of Paris For AC Valhalla
It is first very important to note that this cannot be done on Easy (Skald) difficulty. If you want to save the kings life the game must be on Default (Vikingr) difficulty. This is because you need to reduce the king to a certain amount of HP and Easy difficulty does not give that option. As soon as the fight starts, run behind the king and you will see a lever on the wall. Pull the lever to raise the nearby gate.
Now you need to get the king down to about 33% HP. This is done by reducing the kings stamina next to a lit brazier and then, once he is stunned, pushing the king into the fire. This does around 1/3 of his health. So if you do this twice, he should be down to about 33% HP.
Then lure him into the room where you raised the gate, it has an alter and a large image on the wall. Once again reduce his stamina. You can shoot him in the head or leg to reduce large amounts at once, then just use your abilities. Once he is stunned, instead of pushing him, simply leave the room and pull the lever down again.