What is the lifecycle of a mind flayer? They discover that Billy has been taken over by the Mind Flayer. In season 3, he becomes the unwilling catalyst for the Mind Flayer. "The Stranger" writer Cienna Madrid described the Mind Flayer as one of D&D's "ghastly fiends". He is possessed by the Mind Flayer from the Upside Down in season two. However, the Byers' house phone rings and gives away their location to the Mind Flayer. The mind flayer appears in the first edition "Monster Manual" (1977). The mind flayer appears in the "Monster Manual" for this edition (2000). The mind flayer appears in the "Monster Manual" for this edition (2014). The mind flayer appears in the "Monster Manual" for this edition (2008). Rob Bricken from io9 named the mind flayer as the 9th most memorable D&D monster.