Ancient Spider Dragon

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. 

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes one Web Strand attack, uses its Reel, one Bite attack, one Claw attack, and one Tail attack.

Bite. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) piercing damage and the target must make a DC 21 Constitution saving throw, taking 30 (7d8) necrotic damage and becoming poisoned on a failed save, or half as much damage and does not become poisoned on a successful one. A creature poisoned this way must make another Constitution saving throw at the end of each of its turns (DC 18). If it successfully saves three times, the poisoned condition ends. If it fails its saves three times, it suffers the effects of the Disintegrate spell (the damage is necrotic) and is no longer poisoned. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind.

Web Strand. Ranged Weapon Attack: +14 to hit, reach 120 ft., one creature. Hit: The target is grappled (escape DC 18). The web strand can 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 dragon pulls each creature grappled by it up to 60 feet straight toward itself.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage.

Spider Breath (Recharge 5–6). The dragon exhales dozens of tiny spiders up to 60 feet away. Summon 12 (3d6 + 3) Swarm of Insects (Spiders) in an unoccupied space within range.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The dragon regains spent legendary actions at the start of its turn.

Tail Attack (Costs 1 Action). The dragon makes one Tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 21 Dexterity saving throw or take 12 (1d8 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its speed without provoking opportunity attacks.

Living Hive (Costs 2 Actions). The dragon summons 3 (1d6) Swarm of Insects (Spiders) in unoccupied spaces next to itself.