Homebrew Chiyo's Illithid Species Details

Eldritch Cepholopods
Not much is known about the Illithid, as they and their enemies tend to attack each other on sight. When not hidden in the shadows or depths, the Illithid cuts an imposing figure – tall and emaciated, with a highly-domed head and a cartilage beak hidden behind several facial tendrils - and gripping whatever weapon they favor with three-fingered hands. Unlike most races, Illithid are sexless, reproducing through eggs - or through tadpole transplants - and have no real defining features. Healthy Illithid secrete a thin film of mucus to retain moisture that often has a faint smell, ranging from aromatic tubers like onions to floral scents like vanilla.
Though it’s hard to see in the dark, Illithid skin tones are nearly unlimited, some a single color and some bearing markings. The most important feature is of course their four prehensile face tendrils that they use for holding objects, or for grappling an opponent to devour their minds. Much like a humanoid missing a limb, a missing tentacle can be a source of anxiety from pity or ridicule. 

Underdark Denizens 
Illithid live slightly longer lives that Humans, up to around 140 years. Upon nearly the end of their lives, most Illithid offer themselves to their Elder Brain, letting their leader devour their minds and learn their experiences, which in turn are shared with the entire colony. Illithid are slow to age, and slow to reproduce - one Illithid may only lay one or two eggs in its entire lifetime.
Like most of the Underdark, Illithid are bizarre and strange to anyone on the surface, though there’s many aspects of Illithid life that mirror those above them. While some might make a show of physically eating the brain of their victims, in truth the brain is a poor source of food, being mostly fat and water with no real nutritional value. Illithid feed more like a psychic vampire, and supplement the rest of their diet with foods that are high in carbohydrates and sodium to fuel and retain water in their highly efficient bodies. 
Illithid are intelligence-driven, with a drive to rule over others. Most are raised with the belief that they are the true masters of the universe, and thus have an innate desire to control and dominate. However,  this desire is almost entirely limited to other races, and very few attempt to sabotage their fellows. Illithid work together surprisingly well, as a colony divided is a colony that falls. It isn’t uncommon to see an Illithid alter the source of their goals to help another in times of trouble. This was also extended to the Illithid’s thralls – Illithids often had favorite thralls - doting on them with toys and trinkets, mourning their deaths, and even going mad from loneliness.

Home Is Where The Mind Is 
Illithid live in colonies, cavernous settlements that centered around the deep cave where their Elder Brain was kept safe and secret. Illithid colonies work as a single hivemind, controlled by their Elder Brain, working together to achieve whatever the Elder Brain’s interpretation of the Grand Design was.  Different caverns were carved out of the rock to house Illithids and thralls, or libraries. Illithids spent their daily lives in the company of their thralls, carrying out whatever commands their Elder Brain gave them. Dissent is uncommon in an Illithid colony – if the constant mental monitoring  wasn’t enough to deter it, the fear of being less than 100% efficient was. 
Very few Illithid leave their colonies, and even fewer leave the Underdark. There are exceptions, of  course. Some flee their colonies to escape the grasp of their Elder Brain, some form “Inquisitions” - exploration parties consisting of scholars, psionic warriors, and their thralls. 
Illithid don’t worship in the same way that other races do, but they do have deities: Ilsensine, the Tentacled Lord, patron of evil mages and psionicists, as well as Maanzecorian, the Philosoflayer, the  embodiment of the Illithid concept of complete comprehension of knowledge. Illithid believe that their reward after death is to join with their Elder Brain, melding their experiences with the entire colony. Upon death, an Illithid’s brain is carefully extracted and placed in funerary jars to preserve. This practice is commonly done by a loved one, or a favorite thrall, in private.
While hard to believe, Illithid hold the arts in high regard, decorating their colonies in intricate geometric patterns and tessellations of gold and silver that expressed powerful emotions. Most Illithid musical scores are performed on waterphones and other metal percussions, instruments that could be enhanced by the echoing caves their colonies were housed in while resisting water damage.

Interspecies Relations 
Occasionally, one might find themselves on the same side as an Illithid. Something might have changed the Illithid’s opinion – an inquisitive nature, tactical advantages, or the rare glint of kindness. Regardless, anyone with an Illithid on their side is sure to find that an Illithid is protective of their charges, despite their arrogant and fickle nature.

Names 
Illithid have very little use for names, being part of a psychic collective, and most Illithid have no concept of - or interest in - gender identities. When venturing out and dealing with others, they often take names that originate from the verbal aspects of Deep Speech, using a symphony of grunts and hisses to create a name for themselves.  
Notable Illithid: Galuum, Gideos, Grazilaxx, Methil El-Viddenvelp, Captain N’ghathrod, Nihiloor, Omeluum, Vestress, Yharaskrik, Xetzirbor

Making Your Illithid
As an Illithid, your creature type is Aberration. Your size is Medium, and your walking speed is 30ft. Illithid are considered adults at the age of 40 and live to around 120 years. On average, an Illithid's height is around 6ft to 7ft, and their weight is on average anywhere from 90lbs to 120lbs. While it's common for Illithid to be Lawful Evil, they're never totally bound by a certain alignment.

Chiyo's Illithid Traits

A life underground and a superior mind has granted your Illithid an assortment of inborn abilities.

Ability Score Increase

Your Intelligence increases by 2 and your Wisdom increases by 1.

Languages

You speak Deep Speech, Undercommon, and have a Telepathy range of 60ft

Superior Darkvision & Sunlight Sensitivity

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 can't discern color in darkness, only shades of gray. 
You have disadvantage on attack rolls and on Wisdom (Perception) checks that rely on sight when you, the target of your attack, or whatever you are trying to perceive is in direct sunlight.

Talk to your DM if your Illithid has acclimated to the surface long enough to adjust to the amount of light. If so, you lose Sunlight Sensitivity and your Darkvision range decreases to 60ft

Aberrant Apathy

Deception and Persuasion checks are rolled with disadvantage, and Intimidation checks are rolled with advantage.

Illithid Psionics

You know the Minor Illusion cantrip.
Starting at 3rd level, you can cast the Charm Person spell with this trait.
Starting at 5th level, you can cast Invisibility with this trait.

Once you cast Charm Person or Invisibility with this trait, you can't cast that spell with it again until you finish a short rest. You can also cast either of those spells using any spell slots you have of the appropriate level.

Intelligence is your spellcasting ability for these spells when you cast them with this trait. None of these spells require spell components when you cast them with this trait. In addition, you use your Intelligence modifier when setting the saving throw DC for the spell you cast and when making an attack roll with one.

Spell save DC = 8 + Proficiency Bonus + Intelligence Modifier
Spell attack modifier = Proficiency Bonus + Intelligence Modifier

Devour Mind

You can use your Action to Grapple a creature and use your Bonus Action to drain their mental energy. The creature makes a DC Intelligence save or takes 1D10 plus your Proficiency bonus in psychic damage, and you gain temporary hit points equal to half the amount of psychic damage dealt. Any target that is incapacitated automatically fails. If Devour Mind reduces a target’s Hit Points to 0, the target is killed.

Mind Blast

As an Action, you can emit psychic energy in a 60ft cone. Each  creature in that area must succeed a DC Intelligence save or take 1D8 plus your Proficiency bonus in psychic damage and be stunned until the end of your next turn. Affected creatures can repeat the saving throw at the end of each of their turns, ending the effect on themselves on a success. You can repeat this ability a number of times equal to your Proficiency bonus before requiring a long rest.

Master Mentality

As long as you can see at least two allies within 30ft, you have advantage on saving throws to avoid being Frightened, and disadvantage on those saving throws if you cannot see any.