Using this in the final boss room (The Maw of Despair) is forbidden by the game’s logic. Doing so can corrupt the boss script. Use it in safe zones or cleared rooms only.
Let’s talk about the psychology of horror games. Nightmare Hunter is designed to make you anxious about saving. That anxiety leads to bad decisions: rushing through a trap corridor, fighting a boss at half-health, or skipping a side-quest because you don't want to lose your current save. nightmare hunter full save better