Very close to the end of season 1, episodes 21-24 are adapted beautifully from the manga and are about as perfect as you can get. Askeladd really carries the last four episodes of the season on his back, and this is where his character will reach perfection. Keep in mind what you've learned about Askeladd's lineage and his true goals for the future and his plans for Canute, they'll be important in the coming episodes.