My first exposure to Mind Flayers came from the first campaign of the show Critical Role. I was tangentially familiar with them beforehand, but had little to no knowledge of their lore. Afterwards they became one of my favorite monsters. First introduced in the newsletter of TSR Games, later succeeded by Wizards of the Coast, for the original Dungeons and Dragons. They went on to be in every edition of D&D.
They are psyonic creatures that feed off the brains of sentient being, typically humanoid. Mind Flayers are also subterranean dwellers, making their home in what is called the Underdark. The prey that Mind Flayers deem to be un-worthy of being devoured are turned into mindless slaves to complete whatever task the Flayer commands. What makes them effective monsters is their hive mind society. Each Mind Flayers is connected to a network that is housed in a central Elder Brain. The Elder Brain is compiled of the brains of deceased Mind Flayers, housed in a container filled with some sort of fluid. Rarely, if a Mind Flayer is ever separated from its connection it is possible for it to re-acquire its own independent personality and perhaps even create its own colony sometimes with creatures not of its kind.
They are cunning, powerful, and manipulative monsters that will do whatever is necessary to benefit itself and its hive. Definitely one of the cooler monsters around and a fine choice for a Big Bad of any campaign. Thanks for reading and please be sure to let me know what your favorite monster is! Till next time.