While this is billed as a prequel to Konosuba season 1, it creates a huge continuity problem in that Megumin visited Arkenretia before meeting the others and that makes some of what happened in Konosuba season 2 seem a little off. I really hate people who don't read the existing material before pulling stuff out of their asses.
That said, I gave it a 6, but I have to take this as an alternate version, rather than a true and proper prequel.