You can speak telepathically with any intelligent creature within 120 feet of you.

While a creature that has a brain is restrained, Incapacitated, or Grappled by you, you can use your Action to surround its head with your tentacles and cause 2d10 damage to it. When its health is 0, you may use another action to devour its brain. When you do, you can learn one piece of information that the creature knew. Roll an Investigation check, and on a success, you learn what you want to know if the creature knows it. You also gain 1d4 hit points providing the creature had an intelligence of 5 or higher.

You know the Mage Hand cantrip and .Detect Thoughts which you can cast once per short or long rest. When you reach 3rd level, you can cast the Shield and Catapult once each spell and regains the ability to do so when you finish a short or long rest or can cast detect thoughts as a ritual. When you reach 5th level, you can cast the command and Levitate spell once with this trait and regain the ability to do so when you finish a short rest or levitate can be cast as if it were a ritual. When you reach 10th level, you can cast the Dominate monster and Telekinesis spell once and regain the ability to do so when you finish a Long rest. When you reach 18th level, you can cast plane shift (self only) once and regain the ability to do so after a long rest. Intelligence is your spellcasting ability for these spells and your save DC equals 8 + your proficiency + your intelligence modifier. 

Even those Illithids who have undergone alignment shifts via magical means or mutations struggle to understand the thoughts of less-alien beings. You suffer disadvantage on all Wisdom (Insight) checks with non-aberrations and on any Charisma ability score checks in which you are not proficient in the skill.

Your Intelligence score increases by 2, and your Charisma score increases by 1.

The length at which Illithid lives varies from Illithid to Illithid. Typically the mind flayers live up to 200 years but have been known live significantly longer or shorter lives. All Mind Flayers are mortal and once they become too feeble to serve the colony anymore their body and consciousness are absorbed by the elder brain so little is known how long they can live in a venerable state. Illithids reach full maturity within a week of a tadpole being place into a host body but before that, they could have potentially lived a full life as a humanoid and at least 10 years as a tadpole in the tanks of the elder brain where the knowledge of the Colony is passed onto each individual. Once a full Illithid has formed the being has almost no memory of its previous life but in rare cases, a mind flayer may have flashbacks, vague memories or residual emotions from their previous life. Some may additionally inherit certain traits, personality defects and/or mannerisms from its former host as well. Such Mind Flayers work hard to keep this a secret from their colony as any impurities can lead to their execution.

Illithids have an innate tendency towards evil, domination and order. This is mainly due to the influence of an elder brain that bands all Illithids within 5 miles of it under a single consciousness. Some Illithids are born with mutations that allow them to resist this, or practice forbidden arcane magic to protect them from the elder brain. These Illithids are shunned by Mind Flayer society and often executed for their betrayal. Regardless, the longer an Illithid spends away from the Elder Brain, the more independent a thinker it becomes.

Illithids typically taller and slimmer than the average human (medium) though this can vary based on the Illithids original humanoid form e.g. an Illithid spawned from a Goliath would be unusually taller and more muscular (but most are spawned from intelligent humans or elven host bodies). Your size is Medium.

Your base walking speed is 30 feet.

Illithids are born in darkness and live in the darkest corners of the world; you have superior vision in dark and dim conditions. You can see in dim light within 120 feet of you as if it were bright light, and in darkness as if it were dim light.

You have advantage on Wisdom, Charisma or Intelligence saving throws against magic effects.

Illithids have the special ability to stun their opponents. You can force anyone creature within 30 feet of you to make an Intelligence saving throw. If they fail, they are stunned and must remake the saving throw at the end of their turn. You can only have one creature Stunned in this way at a time, and this ability uses Concentration. When they succeed on the saving throw, they are not stunned, and you cannot use this ability again until you finish a short or long rest. The Save DC for this ability is 8 + Proficiency Bonus + Intelligence Modifier.

You can speak, read, and write Deep Speech, Undercommon and can read and write in Qualith, the Illithid way of writing that can only be read through touch with your tentacles.