Your Intelligence or your Dexterity score increases by 2, and your Charisma score increases by 2.

Accustomed to life in the Underdark, 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 can’t discern color in darkness, only shades of gray.
In bright light your vision is hindered for 30 seconds until your eyes adjust, giving you disadvantage on perception rolls for that time.

You can grasp things with your tentacles; grappling gains the finesse property at level 3. They have a reach of 5 feet, and it can lift a number of pounds equal to your Strength score. You can use them to do the following simple tasks: lift, drop, hold, push, or pull an object or a creature; open or close a door or a container; or grapple someone. As an action, you can use your tentacles along with your body to try to grapple a creature, if you hit, you may make a Beak attack to the grappled target as a bonus action. The Beak Attack deals 1d8 +  Proficiency Bonus + Strength Modifier. Each round the creature is grappled,  you may make additional Beak attacks as a bonus action.

Your DM might allow other simple tasks to be added to that list of options.

When you reduce a living creature within 5 feet of you to 0 hit points, as an action, you may devour their brain completely gaining temporary health equal to the player level + proficiency bonus. If the creature is already grappled by you, you may devour their brain as a reaction In addition, you may receive the answer to one question that the consumed brain knew the answer to. This creature must have a size of Medium or smaller. Alternatively, you may consume the brain of a creature that has died within the last 5 minutes within 5ft. as an action and gain the same benefits.

Your creature type is both humanoid and aberration. Even those Illithids who have undergone alignment shifts via magical means or mutations struggle to understand the feelings of less-alien beings. You have disadvantage on all Charisma ability checks in which you are not proficient in, with the exception of intimidate and deception. If your entire head is exposed, you gain advantage on intimidation checks against creatures unfamiliar with Illithids, or already fearful of Illithids.

In addition, because of your Illilthid physiology and mind, you are resistant to all psychic damage.

You can speak, read, and write Undercommon, speak Deep Speech, and read/write Qualith. Undercommon is a language of commerce among the subterranean races in the same way Common is among the surface dwellers.