That beam attack is massive, like easily the size of multiple buildings. We now know for sure that Lancelot was the one who destroyed all the Chaos monsters in Sistana and it does seem like his power could be comparable to the level that the Sins were on at the start of NNT. I guess the energy arrow is his main magic, presumably a combination of destruction type and search type since it appears to home in on enemies. Other than that he's got his fairy abilities of heart reading and shapeshifting, and perhaps flight as well eventually. A pretty versatile skill set. It's interesting that he opted to launch the attack using Donny's dagger instead of his own bow and arrows.
Lancelot managed to live up to his claim that nobody who has fought him has lived to tell the tale, but I'm guessing Pellegarde's arrival will end up complicating things. It's possible that Pellegarde won't actually try to engage him, but even just witnessing this slaughter would be enough for the Knights of Chaos to learn of Lancelot's existence and perhaps recognize him as one of the Knights of the Apocalypse. It would be easy to make Pellegarde into Lancelot's nemesis among Arthur's knights, similar to the dynamic between Ironside and Percival, but it seems like Lancelot might already have a nemesis. The knight with the star-shaped visor debuted all the way back in chapter 5 and was drawn with almost as much detail as Ironside and Pellegarde. He's bound to be important, but I suppose he doesn't necessarily have to be an enemy. The Q&As mentioned that quite a few of Arthur's knights have defected, so he could very well be a spy that Lancelot wants to get in contact with.
But I can see there being a previous conflict between Liones and Camelot, which could explain why Camelot is apparently gone and the widespread destruction that Arthur was able to use to recruit people like Ardbeg to his side.
Camelot was wiped out by the Demon King when he possessed Meliodas. It was the result of the storm attack that only Ban managed to withstand due to his Purgatory training. That probably also contributed to the hatred Arthur's knights have towards the non-human clans.
Nakaba please hear my plea I want the sunshine Arthur back please please please let it be just Chaos controlling his mind pweeeeeej
I think it's unlikely that Arthur is going to remain evil in the long term. This shift in his personality is very radical and we already know that something shady happened in the past, because Varghese accused Ironside of somehow betraying Arthur despite the fact that Ironside clearly still serves him. I interpret that as Ironside playing a part in Arthur's moral downfall. It also doesn't make much sense to make the strongest character in the series one of the first major antagonists in the series. Then there's of course the more meta reason that the Knights of the Apocalypse are named after King Arthur's most famous knights, which seems like foreshadowing to me.
. Combining heart reading and CQC like this is going to be a problem.
