Ancient Shadow Spider

False Appearance. If the spider is motionless, has its eyes and mouth closed, and has its legs wrapped around its body at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn’t observed the spider move or act, that creature must succeed on a DC 18 Intelligence (Investigation) check to discern that the spider is anything other than it's surrounding material.

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 Shadow Spider makes any combination of two attacks, and one use of Reel.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) piercing damage, and the target must make a DC 19 Constitution saving throw, taking 9 (2d8) poison damage on a failed save, or half as much damage on a successful one. 

If the target is a Large or smaller creature, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the spider. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside of the spider, but takes 21 (6d6) acid damage at the start of each of the spider's turns.

If the spider takes 30 damage or more on a single turn with a creature inside it, the spider must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the spider. If the spider dies, swallowed creatures are no longer restrained by it and can escape from the spider's corpse by using 20 feet of movement, exiting prone.

Leg Attack. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) piercing damage.

Web (Recharge 5–6). Ranged Weapon Attack: +5 to hit, range 120/240 ft., one creature. Hit: The target is grappled and restrained by webbing. As an action, the restrained target can make a DC 12 Strength check, bursting the webbing on a success. Alternatively, the webbing can also be attacked and destroyed (AC 12; 20 hit points; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage). The spider can grapple up to six creatures at a time using its web strands.

Reel. The spider pulls each creature grappled by it up to 60 feet straight toward itself.