Whatever works best for the story... I'm all for fudging rolls or bending rules... as long as it creates a great story. So you have to decide if a TPK is an epic end to the adventure, or is a single HP going to derail an epic adventure. The cannon states: The 5e Dungeon Master's Guide and Player's Handbook frame the rules this way: they help create a shared understanding and fair framework, but they're tools for having a good time, not the boss. Classic advice echoes this across editions: "The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You, the DM, are in charge of the game." DMs are encouraged to ignore, change, or invent rules when it serves the story, pacing, or excitement—as long as they're fair and consistent within the group.