Time for a little Comics 101. There are rules in storytelling, where you set up a scene and you get a payoff scene, etc. When you're watching a TV show or movie and a character has that warm scene with the main character or a love interest and the audience feels great and then suddenly "Uh-oh, I bet that character kicks the bucket soon." A more conventional storytelling method would have had Maris having a nice moment with Lamia earlier on or something, where she'd say "Things have been going really well lately! I think we're finally getting the hang of this magical girl business" or something, to sort of trigger in a reader's mind where they think "Hopefully nothing will offset this momentum they've got going..." I know the rules, but I chose to have this come out of nowhere (though I did have a warm and fuzzy scene with Maris in the last chapter) because I wanted the reader's guard to be down. Things have been pretty peaceful in the last few chapters, but not anymore.
Final fun fact: when I originally wrote this scene and posted it as a novel back in... 2007 or something, Maris was not a very popular character. Imagine my shock when on the official character poll I took for Kickstarter she ranked #3 in popularity... whoops.