The line comes from Matt Berry's performance as Laszlo Cravensworth in What We Do in the Shadows, the television comedy about vampires sharing a house on Staten Island. Berry's larger-than-life, theatrical style has made Laszlo a fan favourite, and his dry, condescending one-liners are frequently clipped for short videos and reaction edits. Thought you were dead fits that pattern perfectly, because it can greet almost any unexpected return with mock surprise. Fans keep reusing it for reunions, comebacks and characters who refuse to stay gone.