The devil has been with Asta since the very beginning – the end of episode 1. We've heard him speak to Asta (or his opponent) a few times before, but now, finally, he gets formally introduced with just one episode left. Interestingly, he actually looks a lot like a less beefcake version of Asta in his true form, including being on the short side.
That's not the only interesting about the devil (whom we know from advertising copy is named Liebe), but it is certainly the most immediately-noticeable one. It also means that what looked like a full-devil version of Asta late in the current opener was actually the genuine devil. At least as importantly, the devil seems to have had peaceable contact with another human in the past, one who – like Asta now – was not willing to believe that he was automatically bad just because he's a devil. That presumably means that Liebe has some reason to hate devils in general, which might explain his dismissive attitude of Zagrad when the latter was defeated by Asta back in episode 120. That backstory looks to be what the series will finish on next week, and I'm going to guess here now that it will be key to Liebe and Asta coming to an agreement as the result of the Devil-Binding Ritual rather than a traditional dominant relationship.
That is the key part of the episode, but hardly the only thing going on and definitely not the episode's biggest plot twist. No, the latter goes to the Heart Kingdom side, which reveals that the elves who persisted after the reincarnation have found a hidden elven homeland in a strong magic region bordering the Heart Kingdom. One revelation at this point is the identity of what is presumably the fourth of the elemental spirits: Dryad, as the spirit of Earth. (Given that the others are Sylph, Undine, and Salamander, the naming convention sounds consistent.) The bigger one, however, is the existence of half-elven children who are direct descendants of Princess Tetia and Licht; in a brand-new flashback, we learn that Nero was able to save the princess and one of her twins back at the time of the massacre, which provides the episode's emotional beat. (And is it any wonder that Tetia might have gone into seclusion after that rather than return to the Clover Kingdom?) Have to wonder how the elves would react if they knew Nero was responsible for that, but it is another reaffirmation that the cost she and Lumiere paid centuries ago paid off in even bigger ways than they realized. Doubtless the elves appreciate the nature of the threat that the Dark Triad poses, so no wonder that they are willing to help Noelle and crew get an elvish-based power upgrade.
Oh, and Nacht is a magnificent prick of a human being, isn't he? While his attitude towards the Black Bulls remaining at their HQ might in some way be meant as a motivation tool, I rather doubt it. He truly does not like any of them, perhaps as an extension of not liking Yami, and that he is a Black Bull only because he needed to be on some Magic Knight squad makes sense. Granted, to a certain extent they deserve it, as their normal mode is sloppy and undisciplined, but he is also being a bit hypocritical, since that undisciplined approach of Yami's is also what allows him to act freely.
Just one more episode to go. My assumption now is that this franchise is probably going to follow a path similar to Naruto, though in this case I imagine that there might be a break of several months to a year in between. An announcement is supposed to be accompanying the final episode, and since this franchise has not done the movie path, I have to think it will be announcing something like that. We'll see next week.