Connor's Young Adult Black Dragon

Acid blood. When the dragon takes at least 7 piercing or slashing damage, blood will spray from the wound. Any creature within 5 feet must succeed on a dexterity saving throw (DC 14) or take 4 (1d8) acid damage.

Acid aura. The dragon has acid floating in the air for five feet around it. If you enter this range, or you start your turn within this range, roll a constitution saving throw, DC 14, or take 4 (1d8) acid damage.

Amphibious. The dragon can breathe air and water.

Bloodied Breath. The first time each day the dragon's hit points drop to one-half or less of its maximum hit points, and its breath weapon is not already charged, its breath weapon recharges.

Corruption aura. While underwater, the dragon passively corrupts the water around it to a range of 10 ft, causing it to become acidic and toxic. Creatures that enter the aura, including when the dragon approaches them, or creatures that start their turn in the aura must succeed on a constitution saving throw (DC 17) or take 11 (2d10) acid damage and become poisoned. 

Caster. The dragon is a third-level spellcaster and can cast the following cantrips from the sorceror spell list. Its casting stat is Charisma and its spell save DC is 15.

Acid Splash, Infestation, Message, Shape Water

The dragon has four level one spell slots and two  level two spell slots. It knows the following spells:

Magic Missile, Sleep, Tasha's Caustic Brew, Maximilian's Earthen Grasp.

Sorcery Points. The dragon has 3 sorcery points. It can spend 1 or more sorcery points to gain one of the following benefits:

Heightened Spell: When it casts a spell that forces a creature to make a saving throw to resist the spell’s effects, the dragon can spend 3 sorcery points to give one target of the spell disadvantage on its first saving throw against the spell.

Empowered Spell: When it rolls damage for a spell, the dragon can spend 1 sorcery point to reroll a number of the damage dice up to 3. It must use the new rolls.

Magic Resistance. The dragon has advantage on saving throws against spells and other magical effects.

Legendary Resistance (1/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes three attacks: one with its bite and two with its claws or tail.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 9 (2d8) acid damage.

Any creature the dragon successfully bites will also take 4 (1d8)  acid damage at the start of their next turn.

Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 16 (3d6 + 6) slashing damage.

Tail. Melee Weapon Attack:  +11 to hit, reach 10 ft., one target. Hit: 10 (1d8+6) bludgeoning damage. Any creature hit in this way must succeed on a DC 18 athletics saving throw or be knocked prone.

Acid Breath (Recharge 5–6). The dragon exhales acid in a 50­-foot line that is 5 feet wide. Each creature in that line must make a DC 17 Dexterity saving throw, taking 51 (11d8) acid damage on a failed save, or half as much damage on a successful one.

Corrupt water. Up to 3 times a day, the dragon may cause water in a 20 foot radius to become acidic and toxic. Creatures that enter the water or start their turn in the water must succeed on a constitution saving throw (DC 17) or take 9 (2d8) acid damage and become poisoned.

The dragon may choose to use its Corrupt Water ability to target a water-based creature within 20 feet. The target must make a constitution saving throw (DC 17) or take 27 (6d8) acid damage and become poisoned on a failed save, or half as much damage on a successful one.

Darkness The dragon can cast Darkness once per day with no material components required. The dragon can see through its own darkness.

Charm aquatic creatures and reptiles The dragon can cast Charm Monster once per day with no material components required. This ability can only affect aquatic creatures and reptiles.

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 15 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.

Degrade equipment. As a reaction to causing acid damage to a target with an attack or spell, the dragon may negate the acid damage instead choosing to degrade the target's equipment, causing AC to decrease by 1 per 20 damage negated until the target's AC is the same as if they had no armor at which point the armor is considered fully degraded. Armor that confers acid resistance is immune to this effect.

The dragon may also choose to degrade the target's wielded weapons, imposing a -1 (per 20 damage negated) to attack rolls made with this weapon, which stacks to a maximum of -4. Weapons which do acid damage are immune to this effect.

Magical equipment degraded in this way will recover at a rate of 1/day. Non-magical equipment will need to be repaired by the appropriate artisan at a cost of 1/5 the weapon's total cost for each point repaired.

Tail whip. As a reaction to an enemy missing with a melee attack, the dragon can immediately attack that enemy with its tail.

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. The dragon makes a tail attack.

Casting (costs 2 actions). The dragon casts a spell. If the spell is a cantrip, this only costs 1 action. Casting as a legendary action depletes the dragon's spell slots as normal.