One player keeps feeling watched, but Perception checks find nothing. After a depleting fight, a Phase Spider emerges, poisons the weakened character, vanishes with its meal. They hunt from the Ethereal Plane, studying patterns, striking when you're vulnerable. Battlefield control spells mean nothing when the spider doesn't exist on your plane. Use them as warnings of planar collapse, ghost predators threatening spirit mediums, or necromancer security systems. Just remember: Phase Spiders let DMs conceal information in ways that feel unfair even when technically legal. Signal the danger. Be fair about the madness.