Illithids usually stand at about the same height as a human, at anywhere from 5 feet to well over 6 feet. Your size is medium.

You Intelligence score increases by 2, and your Wisdom score increases by 1.

 Illithids are genderless and don't reproduce through conventional means. Rather, they lay eggs, from which hatch infant illithids that resemble tadpoles. To grow into a mind flayer, a humanoid must be captured and incapacitated, at which point the tadpole is inserted into the skull cavity (usually through the nose or ear). The tadpole devours the victim's brain, and then over the course of 2d6 days, the humanoid transforms into a fully-grown adult mind flayer. After this transformation, the illithid can live for up to about 125 years. Though the mind flayer usually retains vague memories of its previous life, these memories rarely impact its new life.

 Because of their steady diet of humanoid brains, illithids are almost always evil. A renegade illithid away from the sway of an elder brain might become neutral, but their nature keeps them from ever becoming good. Their culture's hierarchical structure also pushes them strongly toward law.

Your base walking speed is 30 feet.

Accustomed to life in the Underdark, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.

As an illithid, humanoid brains make up a large part of your diet. You can survive on other foods, but none of them provide the psychic stimulation that brains do. When you devour the brain of a humanoid, you experience euphoria along with its memories, personality, and fears. At the DM's disgressions, you might retain a bit of useful information from the feast, but usually the things you experience are fleeting. Watching the feast is disturbing and scarring to other humanoids.

You have advantage on all Intelligence, Wisdom, and Charisma saving throws against spell effects.

You can speak, read, and write Common and Deep Speech. You can also read Qualith, the special written language of mind flayers. To read it, you must use your tentacles to feel the inscription, reading the psychic imprint left by another illithid. For more information on Qualith, refer to the 5th Edition Monster Manual and Volo's Guide to Monsters.

You know the mage hand cantrip. When you cast the spell using this trait, the hand is invisible. When you reach 3rd level, you can cast the detect thoughts and levitate spells once each with this trait and regain the ability to do so when you finish a long rest. Intelligence is your spellcasting ability for these spells.

You can communicate telepathically with other creatures you can see within 60 feet of you. You don't need to share a language for it to understand you, as your telepathy takes the form of advanced concepts and associations, which other humanoids interpret in their own language. For more information on Mind Flayer telepathy, refer to Volo's Guide to Monsters.