The ending of 'Nightmare' hits like a freight train of emotions. Victor finally confronts the demon king in a brutal final battle where both are pushed to their absolute limits. Using the forbidden technique he learned from the ancient tome, Victor sacrifices his own memories to unleash a soul-shattering attack that obliterates the demon king's physical form. In the aftermath, the nightmare realm begins collapsing as Victor wakes up in his bed, drenched in sweat but alive. The twist comes when he notices the demon king's sigil faintly glowing on his palm - suggesting the fight isn't truly over. The last scene shows Victor smirking at this revelation, hinting he might actually embrace the darkness rather than fear it.
For those craving similar psychological thrillers with ambiguous endings, I'd suggest checking out 'The Silent Patient' or 'House of Leaves'. Both play with reality in equally disturbing ways.