It's just a quick pep talk from "Iron Fist" Ketil towards his men, who are awaiting the arrival of Canute and his army. The short scene immediately follows the ending of episode 18, to fit the standard TV run time, they obviously had to cut corners and exclude this little scene from the anime. It's not necessarily a "must see", I think everything understood the importance of what was to come, though it's only 90 seconds, so if you're a big Vinland Saga fan or a completionist, it'll only take a little of your time. The OST is very good as is usually the casewith this show, fits very well with the motivational speech. Really need to sit down and listen through their soundtracks one day.

Episode 18.5 of Vinland Saga is a 90 second scene that could have been in the main anime but was left out. The story is about Ketil giving a speech to the men that he gathered. This scene explains the start of episode 19 and is actively discussed in episode 19. It shouldn't have been left out of the anime and is definitely worth watching or some of the things that are said in episode 19 won't make much sense. The animation, voice acting and music are all the same as in the main anime. Overall this scene is definitely worth watching since it addscontext that is talked about in episode 19 and it won't make as much sense without watching this scene. It's also only 90 seconds so it won't take much of your time.