Black Dragon Arachnomancer

Amphibious. The dragon can breathe air and water.

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

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

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

Multiattack. The dragon makes three melee attacks. One attack can be replaced by a use of Spellcasting or Web.

Bite (Dragon Form Only). Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) acid damage.

Claw (Dragon Form Only). Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail (Dragon Form Only). Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Spider Bite (Spider Form Only). Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 31 (7d8) 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.

Poisonous Touch (Humanoid Form Only). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 35 (10d6) poison damage.

Web (Spider Form Only; Recharge 5–6). Ranged Weapon Attack: +8 to hit, range 30/60 ft., one target. Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 15 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 10; hp 5; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).

Spellcasting. The dragon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 16):

At will: dancing lights, mage hand

1/day each: darkness, dispel magic, etherealness, faerie fire, insect plague, invisibility

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Acid Breath. The dragon exhales acid in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) acid damage on a failed save, or half as much damage on a successful one.

Poison Breath. The dragon exhales poisonous gas in a 60-foot cone. Each creature in that area must make a DC 18 Constitution saving throw, taking 56 (16d6) poison damage on a failed save, or half as much damage on a successful one.

Humanoid Form. The dragon magically transforms into any humanoid that is Medium or Small (usually a dark elf/drow), while retaining its game statistics (other than it’s size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

Spider Form. The dragon magically transforms into a Large spider, remaining in that form for up to 1 hour, or back into its true form. Its statistics, other than its size, are the same in each form. It can speak and cast spells while in spider form. Any equipment it is wearing or carrying in Dragon or Humanoid form melds into the spider form. It can’t activate, use, wield, or otherwise benefit from any of its equipment. It reverts to its Dragon form if it dies.

Summon Spiders (1/Day). The dragon magically calls 2d4 swarms of spiders. The called creatures arrive in 1d4 rounds, acting as allies of the dragon and obeying its spoken commands. The beasts remain for 1 hour, until the dragon dies, or until the dragon dismisses them as a bonus action.

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.

Detect. The dragon makes a Wisdom (Perception) check.

Attack. The dragon makes one melee attack.

Cast a Spell (Costs 2 Actions). The dragon uses Spellcasting.