Ancient Black Dragon

The ancient black dragon can breathe air and water.

4/day (5/day in lair). If the ancient black dragon fails a saving throw, it can choose to succeed instead.

The ancient black dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Melf's Acid Arrow (level 4 version).

Melee Weapon Attack: +15 to hit, reach 5 ft.. Hit: 17 (2d8 + 8) slashing plus 9 (2d8) acid damage.

Recharge 5–6. Dexterity Saving Throw: DC 22, each creature in a 90-foot-long, 10-foot-wide Line.
Failure: 67 (15d8) acid damage.
Success: Half damage.

1 use per day. The ancient black dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 21, +13 to hit with spell attacks):
At Will: Detect Magic, Fear, Melf's Acid Arrow (level 4 version)
1/Day Each: Create Undead, Speak with Dead, Vitriolic Sphere (level 5 version)

Dexterity Saving Throw: DC 21, one creature the ancient black dragon can see within 120 feet.
Failure: 33 (6d10) poison damage, and the target has Disadvantage on saving throws to maintain Concentration} until the end of its next turn.
Failure or Success: The ancient black dragon can't take this action again until the start of its next turn.

The ancient black dragon casts Fear. The ancient black dragon can't take this action again until the start of its next turn.

The ancient black dragon moves up to half its Speed, and it makes one Rend attack.