An oblex can live indefinitely, only being killed by an underestimated target or being destroyed as a threat to the very mind flayers that created it. In rare cases of starvation, death will also occur.

This creatures size is Medium

Having been birthed by the mind flayers you share much of their philosophy. Your alignment is Neutral Evil

Your Constitution  score increases by 2 and your Wisdom score increases by 1

You can speak common, undercommon, and the primary language of whatever humanoid you are currently taking the form of

You do not need to sleep

Your creature type is Ooze

Non-magical restrants do not work on you, as you can just slip right out. You can also slip through a space as small as 1 inch without squeezing. 

You gain proficiency in the Deception skill

This creature may take the physical appearance and voice of any humanoid sized medium or smaller that it has used its Devourer of Memories trait on. It may change its form at will, but the process takes 1 minute during which the Oblex is exposed.

When you die or fall unconcious, you revert to your true gelatinous form

Gain advantage on strength checks made to grapple

After taking fire damage you gain disadvantage on attack rolls and ability checks until the end of your next turn

The oblex can make this attack against a creature that is stunned, grappled, restrained, asleep, or incapacitated. Your true nature is also revealed during this, as colored tendrils extend from your body and face to wrap around the targets head. 

The target is forced to make a dc 15 wisdom save

On a failure it takes 2d8 psychic damage and all attack rolls and ability checks are reduced by 1d4 for an hour. You also learn of any resistances and immunities the target may have. If performed on a humanoid you also learn of any proficiencies it may have and what languages it knows. Constructs, plants, undead, and oozes automatically succeed on this save.

On a success it takes half damage and you learn nothing

After 5 successful uses you must split, creating an oblex spawn or lose proficiency in deception, as you begin to grow more bulbous as you feed. If after 10 feedings you have not split, you grow a distended blob from which your body extends, gaining disadvantage on all deception checks until the split is performed, which after 10 times would spawn an adult oblex. This can be counteracted through use of minor illusion, major illusion, or seeming.

If a creature would drop to 0 hp from this attack, as long as it is medium size or lower, you can completely engulf and devour the creature, leaving no trace behind.

The damage for this increases by 1d8 at levels 4, 9, and 13

For every 5 days you go without feeding, you gain 1 level of exhaustion, that will not go until you feed. If you reach 3 levels you cannot properly maintain your form and are revealed. 

You may use this trait a number of times equal to your proficiency bonus per long rest