A critical hit against a Selphid will deal damage to it directly, ignoring any temporary hitpoints from the takeover ability.

As a selphid the nerves in your body are essentially dead, or at least weakened. You gain disadvantage on Wisdom (Perception) Checks that rely on smell or touch.

The difference between your true form and body allows you immunity to some touch-ranged spells. Most damage dealing spells like shocking grasp and burning hands will still deal damage to the body. But vampiric touch or Cure wounds, would not work unless the caster touches the selphids true form. Which spells fall under this ability is up to the DM.

You gain a +2 to wisdom, intelligence, or charisma, while you gain a +1 depending on the body you inhabit. (Detailed in the takeover trait)

While outside of a body you gain a -6 to strength. So long as you are within a functioning body you have an ordinary strength.

Selphids age and mature at the same rate as humans

Your movement speed while out of another body is 5ft

Selphids themselves are rather light and can fit easily inside of other creatures, your true size is small.

Selphids can attempt to take over the body of another dead or incapacitated creature with a humanoid skeletal structure and a size of medium or small. With a minute of work and a medium medicine check, the selphid can take the body of the other creature. If the creature is still alive it can make a series of constitution saves against the selphids medicine checks. (The number of checks can be decided by the DM, a human commoner will be easier than a mind flayer warlord) If it fails this check it is essentially dead, and it loses control of its body.

The selphid gains the movement speed of the creature it takes, and also its primary racial bonus. For example, as a tiefling has a +2 to charisma, then a selphid in a tieflings body might gain a +1 to charisma. Bonuses to Wis or Int don't apply for obvious reasons, and if that is the race's primary bonus then the selphid inherits the secondary bonus instead.

The selphid also inherits any biological racial abilities like natural armor weapons, or something like a lizardfolks ability to hold their breath. Abilities that are skill-based are not gained, like a goblins nimble escape, or a centaur's proficiency in nature.

When a selphid possesses the body of another creature it gains temporary hit points equal to 150% of the selphids max health. While not in another creature's body the selphid only has a fifth of its max health. These temporary hit points can not be regained except by possessing a new body, and if a selphids temporary hitpoints ever drop to 0 its body is considered broken beyond repair and the selphid must find a new body.

As long as a selphid still has temporary hitpoints left from its "Takeover" trait, it can attempt to push its body beyond its limits for incredible feats of strength. As a bonus action, a selphid can give itself a strength and dexterity score of 22, and a 1d6 to all of their weapon attacks. [This damage increases to a d6 at 5th level, a d8 at 11th level and a d10 at 17th level]

For every turn the selphid uses this effect it will take 1d4 force damage from the strain the rampage takes on their muscles. [This damage increases to a d6 at 5th level, a d8 at 11th level and a d10 at 17th level]

Selphids can stop rampaging as a free action. This effect automatically ends if the selphid runs out of temporary hitpoints.