Ancient Phase Spider

Ethereal Jaunt. As a bonus action, the spider can magically shift from the Material Plane to the Ethereal Plane, or vice versa.

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

Web Walker. The spider ignores movement restrictions caused by webbing.

Multiattack: The  Ancient Phase Spider can make two Bite attacks, or one Bite attack and one Web attack.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 16 (2d10 + 5) piercing damage, and the target must make a DC 18 Constitution saving throw, taking 36 (8d8) poison damage on a failed save, or half as much damage on a successful one. If the poison damage reduces the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.

Web (Recharge 4–6). Ranged Weapon Attack: +10 to hit, range 30/60 ft., one creature. Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 16 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 14; hp 20; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).