Your natural size is Tiny, however, you begin in a human body, thus the size of your host is Medium by default. Your host body's size may change if you successfully take control of another humanoid body via Body Thief at some later time.

A rogue intellect devourer's lifespan is indefinite, but otherwise determined by that of its host. If the host's body dies, the rogue intellect devourer may be able to take control of another host body if it successfully performs a Body Thief attempt.

Most intellect devourers are spawned and live among the Illithid and other intellect devourers and Illithid slaves. Culturally, their kind are inclined toward lawful and evil behavior, but a very small number of intellect devourers resist the influence of their origins. Even so, chaotic and good behaviors are rarest of all. A rogue intellect devourer is generally on the run from any and all who are psychically connected to the Illithid and their agents, and self-survival generally outweighs all other motivators.

As an intellect devourer, your base walking speed is 40 ft., however, your host's walking speed is 30 ft. by default. Movement speed(s) will be overridden if you possess another form via Body Thief. Taking possession of an aaracokra's body, for example, would grant you flight speed of 50 ft., but restrict you to a walking speed of 25 ft.

You can speak, read, and write Common, Deep Speech, and one other language of your choice.

Your Intelligence increases by 2. Your Wisdom score or Charism score increases by 1.

Rogue intellect devourers can invoke blindsight of 15 feet while in their host body. This lasts for one minute once per long rest. If blindsight is available to the rogue intellect devourer through any other means, the duration and distance are doubled.

You can speak telepathically to any creature you can see within 60 feet of you. You don’t need to share a language with the creature for it to understand your telepathic messages, but the creature must be able to understand at least one language or be telepathic itself.

As a bonus action when you’re speaking telepathically to a creature, you can give that creature the ability to speak telepathically to you until the start of your next turn. To use this ability, the creature must be within 60 feet of you and be able to see you.

The intellect devourer targets one creature it can see within 15 feet of it that has a brain. The target must succeed on a [DC 8 + INT modifier + proficiency score] Intelligence saving throw against this mental attack or take 1d8 psychic damage. Also on a failure, roll 3d6; if the total equals or exceeds the target’s Intelligence score, the target is stunned for a minute. This ability may be used once per short or long rest. At level 5, the damage increases to 2d8. At level 11, the damage increases to 3d8. At level 17, the damage increases to 4d8.

The rogue intellect devourer initiates an Intelligence contest with an incapacitated humanoid of no greater than medium size within 5 feet of it that isn’t protected by protection from evil and good. If it wins the contest, the rogue intellect devourer magically consumes the target’s brain, teleports into the target’s skull, and takes control of the target’s body. The rogue intellect devourer retains its Intelligence, Wisdom, and Charisma scores, as well as its understanding of Deep Speech, its telepathy, and its traits. It otherwise adopts the target’s physical statistics. It knows everything the creature knew, including languages. Innate racial abilities are retained.

If an intellect devourer's host body dies, the devourer must leave it. A protection from evil and good spell cast on the body drives the intellect devourer out. By spending 15 feet of its movement, the intellect devourer can voluntarily leave the body, teleporting to the nearest unoccupied space within 5 feet of it. The body then dies, unless its brain is restored within 1 round.

Successful completion of Body Thief results in four (4) levels of exhaustion for the rogue intellect devourer until it can complete a long rest. A level of exhaustion is removed for each subsequent long rest, Greater Restoration spell, or Potion of Vitality.