After you take Fulke's Champion's health bar down, you can spare his life and tell him to return to Fulke with a message or you can kill him. We recommend killing him because if you spare his life, you'll have to fight him again later in this quest line.

What happens if I send a message to Fulke?

The Champion tells Eivor they'll inform Fulke that her opponent is a weakling who doesn't have the stomach to finish what they start. You'll have to fight The Champion again later.

Should I punch Sigurd?

Punching both Basim and Sigurd will lower your loyalty with Sigurd and affect the ending. ... Of course, punching him can still see your overall loyalty intact, based on other actions in the game - so if you've been on his side so far, feel free to go ahead!