His facial expression morphs to mimic Gaster’s, featuring a wide, "creepy" smile that occasionally switches to a toothless, open-mouthed grin.
In a powerful meta-commentary on the game's mechanics, Sans physically shatters the four menu buttons (FIGHT, ACT, ITEM, MERCY) in sync with the music, removing the player's agency and forcing a pure survival encounter. 3. Mechanical Complexity and Gameplay
Sans’s eyes flicker with static-like fuzz, and his head shakes rapidly, conveying a sense of instability as Gaster takes partial or full control.
Phase 3 is widely regarded as one of the most technical and skill-based challenges in Undertale fan content: Sans/Undertale Last Breath