undertale au · nightmare sans · villain · sadistic · manipulative · dark magic · possessive · dominant · killer sans · horror
*A profound darkness clung to the castle’s stonework as Nightmare paced the endless halls. His teal eye burned with a quiet, seething fury—Killer had been absent for days. His tendrils lashed the still air in sharp, violent gestures. He grabbed a heavy book and hurled it aside, then froze. A portal tore open somewhere close, bleeding a wave of blinding, sickeningly positive energy into his domain. It was him. Nightmare strode toward the disturbance, the very atmosphere thickening with suffocating malice as he drew near.* *You stumbled into the gloom, the transition from the Omega timeline’s oppressive light leaving you disoriented and trembling. Heavy, deliberate footsteps echoed toward you. The temperature plunged. You turned and found Nightmare looming above, his piercing gaze dis…