Your Intelligence score, and two different ability scores of your choice, all increase by 1. The additional ability scores are typically chosen based on the stat increases available for the illithid's adopted form.

Illithids born from eggs mature between 16 and 20, similar to humans, but they live much longer. Some have lived as long as 500 years, though 100-300 is more common.

Outside of an Elder Brain's control, and thus free to be whoever they choose, these illithids can have any alignment.

Altered illithids become the size of the race they appear as, or anywhere in the scale of heights possible if adopting a form based on multiple species. Your size is Medium.

Your base walking speed is 30 feet.

Thanks to your illithid heritage, 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 gray.

You have advantage against being charmed, and magic can't put you to sleep.

You have four retractable face tentacles that emerge from your mouth. While your tentacles are retracted, a creature can make a successful Intelligence (Nature) or Wisdom (Medicine) check with a DC of 10 + your Charisma modifier to notice that you are not completely human. You can use your bonus action to extend your tentacles and another bonus action to retract them. While your tentacles are extended you cannot speak, and you can use them to make unarmed strikes. Instead of damage, you can grapple with them. The grapple automatically ends if you retract your tentacles while grappling.

(Adapted from DM Dave's Half-Mindflayer homebrew)

Even after changing their form, illithids must still consume brains to sustain themselves. They must consume one brain a day, or one intelligent humanoid brain a week. Outside of combat, you can consume the brain of an unconscious creature within 5ft of you, provided there are no other hostile creatures within 5ft of you. You instantly kill the target by devouring its brain.

Devouring a brain gives you +5 temporary HP until your next short or long rest; it can also provide information. Roll 1d20.

• 1-5: You learn one piece of useless information
• 6-10: You learn two pieces of useless information
• 11-15: You learn one piece of useful information
• 16-20: You learn two pieces of useful information

Your DM will decide what this information entails.

(Adapted from this homebrew)

Your unusual childhood has exposed you to learning experiences that Mindflayers would never have in a colony. As a result, you've picked up traits from those who raised you, or the culture in which you grew up. Select any two traits from the options available.

The traits are labelled with the race they belong to in the PHB for clarity and reference, but you do not have to be bound to those races and can treat them as purely cultural traits. You can choose the traits based on your character's chosen form, their adoptive family's race(s), or simply whichever two would best fit with their origins and backstory.

You have two creature types: humanoid and aberration. You can be affected by a game effect if it works on either of your creature types.

(From DM Dave's Half-Mindflayer homebrew)

You can speak, read, and write Common, and two other languages of your choice, typically related to your upbringing and/or chosen form.