Alien Symbiote

 Amorphous. The symbiote can move through a space as narrow as 1 inch wide without squeezing.

Spider Climb. The symbiote can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Amphibious. The  symbiote can breathe air and water.

Regeneration. The symbiote regains 10 hit points at the start of its turn. If the symbiote takes fire damage, this trait doesn't function at the start of the symbiote's next turn. The symbiote dies only if it starts its turn with 0 hit points and doesn't regenerate.

 Alien Nature. A symbiote doesn't require air, food, drink, or sleep. Just a living host to "feed" off it's bodies chemistry

Psudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) damage.

Bonding (Recharge 3). One unwilling creature that the symbiote can see within 5 feet of it must succeed on a DC 18 Charisma saving throw or become a host for the symbiote. If willing the creature automatically fails; The symbiote now controls the body but doesn't deprive the target of awareness. The symbiote can't be targeted by any attack, spell, or other effect, except area effects, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunities. It otherwise uses the possessed target's statistics.

Wile bounded the host receives  benefit and hindrance s depending on the host including: +4 to either strength, dexterity, or constitution, +2 to one of the other two not picked. -2 wisdom, +20 ft movement to walking and swimming speeds, 2 points of regeneration a round, cannot be flanked by less than 4 creatures, and no longer needs to breath. Also the symbiote will replace any missing body parts and temporarily halts any dieses wile bonded.

The possession lasts until the body drops to 0 hit points, the Symbiote ends it as a bonus action, or the symbiote is forced out by an effect like the dispel evil and good spell. When the possession ends, the symbiote reappears in an unoccupied space within 5 feet of the body. The target is immune to this symbiote's bonding for 24 hours after succeeding on the saving throw.

Innate Spellcasting. The symbiote's spell casting ability is
Charisma (spell save DC 14). The symbiote can innately cast the
following spell, requiring no material components: 2/short rest each: alter self