Adult Black Dragon

Huge Dragon (Chromatic), Chaotic Evil

AC 19     Initiative +12 (22)
HP 195 (17d12 + 85)
Speed 40 ft., Fly 80 ft., Swim 40 ft.

Ability Scores
STR 23 (+6) Save +6    DEX 14 (+2) Save +7    CON 21 (+5) Save +5
INT 14 (+2) Save +2    WIS 13 (+1) Save +6    CHA 19 (+4) Save +4

Skills Perception +11, Stealth +7
Immunities Acid
Senses Blindsight 60 ft., Darkvision 120 ft.; Passive Perception 21
Languages Common, Draconic
CR 14 (XP 11,500, or 13,000 in lair; PB +5)

Traits
Amphibious. The dragon can breathe air and water.
Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Actions
Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Acid Arrow (level 3 version).
Rend. Melee Attack Roll: +11, reach 10 ft. Hit: 13 (2d6 + 6) Slashing damage plus 4 (1d8) Acid damage.
Acid Breath (Recharge 5–6). Dexterity Saving Throw: DC 18, each creature in a 60-foot-long, 5-foot-wide Line. Failure: 54 (12d8) Acid damage. Success: Half damage.
Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 17, +9 to hit with spell attacks):
At Will: Acid Arrow (level 3 version), Detect Magic, Fear
1/Day Each: Speak with Dead, Vitriolic Sphere

Legendary Actions

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Cloud of Insects. Dexterity Saving Throw: DC 17, one creature the dragon can see within 120 feet. Failure: 22 (4d10) Poison damage, and the target has Disadvantage on saving throws to maintain Concentration until the end of its next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Frightful Presence. The dragon uses Spellcasting to cast