Arachno-Dragon

Armor of Spiders (1/Day). If Arachno-Dragon is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 100 hit points. In addition, its remaining spiders immediately swarm over its body to protect it, granting 40 temporary hit points for each swarm.

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

Immutable Form. The Arachno-Dragon is immune to any spell or effect that would alter its form.

Magic Resistance. The Arachno-Dragon has advantage on saving throws against spells and other magical effects.

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

Web Walker. The Arachno-Dragon ignores movement restrictions caused by webbing.

Multiattack. The Arachno-Dragon makes three attacks: one Bite attack and two Web Strand attack or two Claw attacks.

Bite. Melee Weapon Attack: +14  to hit, reach 5 ft., one target. Hit: 19 (2d10 + 7) piercing damage, and the target must make a DC 17 Constitution saving throw, taking 17 (3d10) poison damage and is poisoned on a failed save, or half as much damage on a successful one. If it fails the saving throw by 5 or more and is already poisoned, the target becomes paralyzed, else the target is poisoned. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Claws. Melee Weapon Attack: +14  to hit, reach 5 ft., one target. Hit: 15 (2d6 + 7) slashing damage.

Web Strand. Ranged Weapon Attack: +12  to hit, reach 120 ft., one creature. Hit: 10 (1d8 + 5) acid damage and is restrained. The target takes another 10  (1d8 + 5) acid damage at the end of each of the creatures turn while it is restrained. The web strand can be attacked and destroyed (AC 12; 20 hit points; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

The Arachno-Dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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.

Web Spray. The Arachno-Dragon and all of it's spiders shoots web, covering the occupied area of the spiders. This area is difficult terrain.

Claws. The Arachno-Dragon makes one attack with her claws.

Swarm Breath (Costs 2 Actions). Arachno-Dragon spews out two swarms of spiders that appear in unoccupied spaces within a 45 ft cone of the dragon. The bite attack of these spiders gain an additional +3 to hit and causes a DC 13 Con saving throw or become poisoned on a hit.