The villain staged a battle they could not win. A dragon, perhaps. The party was cornered. They needed to retreat. But to retreat, someone had to stay behind to hold the line.
Choose your Demon Lord form wisely. Three archetypes exist: hero party must fall guide full
If you mean the The Hero Party Must Fall (or similar titles like Hero’s Party Must Fall ), here’s a general full guide structure: The villain staged a battle they could not win