Alien Symbiote Monster

Spider-Climb.The Alien Symbiote naturally has the Spider-Climb ability and can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Possession Save.  Once per day the host the Symbiote has possessed must make a DC18 wisdom saving throw

Additionally , Any time the Symbiote Monster suffers Fire or Thunder damage, the possessed creature makes a Wisdom saving throw DC18 with advantage to attempt to break free of the creature.

Slash Attack. The Alien Symbiote can shape it's liquid form into a slashing, blade-like weapon. +9 to hit, reach 5 ft., 1 target. Hit: 10(1d6+4) Slashing damage. 

Prehensile Tendrils.The Alien Symbiote can create long, web-like prehensile tendrils that it can use to lift objects, grapple opponents and climb ledges. they can also be used as ranged bludgeoning weapons.

Prehensile Tendril Weapon Attack: +9 to hit, range 10/30ft., 1 target. Hit:12 (1d8+4) Bludgeoning damage.

Venomous Bite. The Alien Symbiote can bite any creature that it has successfully grappled within melee range. if a successful grapple is made with the tendrils, the Symbiote must use it's bonus action to pull the grappled creature to within melee range before it can perform the bite attack. 

Venomous Bite Attack. +9 to hit, reach 5 ft., 1 target. Hit: 18(2d8+4) Piercing damage. The victim must make a Constitution saving throw DC 18 or suffer an additional 1d4 Poison Damage and suffer from the "Poisoned" effect.