I recently got a chance to demo a lesson to two undergraduate classes of Secondary English Education majors on how one could use Undertale to teach practice close reading activities using alternate forms of text.
The short version of the lesson is that students were given a "Clue Sheet," and were instructed to write down anything that could possibly be an important clue to figure out what was going on in the section of the game Undertale that I played in front of the class. I had a pre-saved file about 25 minutes into the game, and the students directed me to explore and examine parts of a mysterious house. I let them know going into the lesson that their clues would be very important, because they would be used to make a decision. The exploration of the house and clue gathering takes about 10 minutes, and then students shared their different ideas of what was going on. One student read all of the text aloud, which seemed to help with engagement. We then moved onto the final part of the gaming session, where students had to vote and chose whether to fight or spare the character Toriel. Students used their clues to make arguments, and attempted to sway their classmates to their points of view. After the decision is played out, we debriefed a bit, and if this was done in an actual high school classroom, I would have had the students turn their experiences into some form of an argumentative essay, or at least a thesis/claim statement.
Out of the 30 or so students across the two college classes, the large majority were female students who did not identify as "gamers," but they almost all really got into the game, wanting to figure out what was going on, with some very passionately arguing for fighting/sparing Toriel. Even the professor, who is almost 60, got really into it! The whole thing took about 25-30 minutes, but you could easily stretch it out to a full class period by adding more formal writing to the end of it. I would consider this game appropriate for 9th grade+, and it's the kind of thing that anyone can find engaging. Undertale is just a great game in general, and I'd eventually like to develop a unit around playing the entire game as a class. I actually got inspired to try turning the game into a lesson because of some 9th grade students that I had last year who loved the game.
I attached a PDF of a sample lesson plan, which also has the "Clue Sheet" attached. It has Oklahoma State standards in it, but you can easily change the standards to fit into any state (or Common Core).