One of the most popular monsters in Dungeons & Dragons is the mind flayer, and for a good reason. Beyond its name recognition from Stranger Things, the mind flayer has been a mainstay of D&D for several editions. The engaging lore behind the monster stokes the imagination with unique abilities not seen elsewhere in the bestiary and a reputation for deadliness. In fact, many Dungeon Masters may want to integrate a mind flayer into their campaign, but if so, they may want to do so with careful consideration. The mind flayer is a whole lot harder to pull off than it seems.
Much of the mind flayer's lore makes it the perfect villain to a campaign. Once masters of an illithid empire, mind flayers share in a hive mind mentality and worship a central elder brain as they seek conquest over the free will of the other races. Their origins tied in with the evocative Underdark, and with each one carrying a personal appetite to eat brains, all of the lore surrounding them sets them up as the ideal evil for everything from a side quest to being the Big Bad Evil Guy of a whole campaign.
In terms of their actual stat block in the Monster Manuel, mind flayers inhabit the difficult-to-balance middle-ground. At a Challenge Rating of seven, they could be a deadly encounter for a four-person party of level fours or an easy encounter for a four-person party of level eights. What's even more complicated is that the CR system does not accurately encapsulate the mind flayer's abilities because it fights in such a unique way high dependent on its environment.
The tentacle-faced mind flayers of D&D have everything from long distance to mid-range to melee abilities. Innate abilities and spellcasting like telepathy, detect thoughts and levitate set them up to navigate almost any environment. They can cast dominate monster to turn a party against itself, and their mind blast ability can potentially stun an entire party at range all at once. In melee, their tentacles allow them to damage and grapple an opponent. Once grappled, they can use extract brain to deal a massive amount of damage and an instant kill. Similarly, planar shift can be used offensively to remove a PC from combat completely.
With relatively few hit points or a poor Armor Class, a mind flayer encounter can be too easy if DMs aren't careful. The trick comes in planning out the environment well and giving the mind flayer the appropriate number of minions. As masterminds, mind flayers lend themselves to psychically controlling other creatures like ogres. Their telepathic and evil ways would lend them toward an alliance with demonic entities like a succubus. All of these factors can drive up the CR of the encounter pretty easily, so constantly keep balance in mind for how they're integrated.
Most importantly, keep in mind the result of the mind flayer's combat options and consider how fun they may be for your players. The mind blast ability's stunning option is one of the mind flayer's most powerful options, but on a meta-level, it can also create a boring encounter where stunned players aren't doing anything except saving throws on their turn. Similar to using the planar shift offensively, try to reserve such options for when the mind flayer truly needs them, and only under a circumstance where combat won't drag out too long after they're employed.
Setting up those melee attacks with the tentacles and brain extraction really gives mind flayers their flavor, but they are hard to pull off if the players are unhampered. The key to giving the mind flayer allies is so that those allies can occupy the rest of the party as the mind flayer tries to devour a lone PC's brain. Such an instance can be a dramatic moment filled with adrenaline for the players as they struggle to save their ally, and it's a unique experience only a mind flayer can create.
In the end, recognizing what is unique about the mind flayer is the key. Appreciate the monster for its own merits rather than throwing it in as a generic monster and a session built around it can truly shine. If done right, combat with a mind flayer can be downright mind-blowing.