Corrupted 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 Sense. While in contact with a web, the spider knows the exact location of any other creature in contact with the same web.

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

Multiattack. The Corrupted Phase Spider makes 3 attacks, one with its Bite and two with its massive legs.

Massive Legs. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit:  23 (3d10 + 6) bludgeoning damage.  The target must make a DC 15 Strength saving throw or be knocked back 10 ft. and knocked prone.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 13 (2d10 + 6) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 27 (6d8) 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.

Defender.   Other creatures provoke an opportunity attack from you when they enter your reach.  If a creature within the Corrupted Phase Spiders reach takes the Disengage action and moves out of reach, an opportunity attack can be made with Disadvantage.

Call the brood.  The Corrupted Phase spider makes a deep reverberating sound that shakes the very ground.  1d4 Giant Spider enter combat from a suitable location.  Taking the current place in the initiative and immediately getting a full turn.

Crazy legs.  The Corrupted Phase spider sends it's Massive Legs in all directions, resulting in a single Massive Legs attack to all creatures within 10 ft. of the Corrupted Phase Spider.