Ancient styx dragon

Amphibious. The styx dragon can breathe air and water. 

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

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

Magic Weapons. The styx dragon’s weapon attacks are magical.

Secure Memory. The styx dragon is immune to the waters of the River Styx as well as any effect that would steal or modify its memories or detect or read its thoughts.

Innate Spellcasting. The ancient styx dragon's innate spellcasting ability is Charisma (spell save DC 20). The ancient styx dragon can innately cast the following spells, requiring no material components:

3/day each: control water, darkness (radius is doubled), fog cloud

1/day each: abi-dalzim's horrid wilting, feeblemind, hold monster, stinking cloud

Multi attack. The adult styx dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its tail spike.

Bite. Melee Weapon Attack: +15 to hit, reach 20 ft., 1 target. Hit: 19 (2d10 + 8) piercing damage and the target must make a DC 23 Constitution saving throw or be afflicted with flesh rot in the next hour. 

Tail spike. Melee Weapon Attack: +15 to hit, reach 20 ft., 1 target. Hit: 15 (2d6 + 8) piercing damage. If the target is huge or smaller, it is Grappled (escape DC 18) and Restrained until the grapple ends. The styx dragon has two tail spikes, each of which can grapple one target.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., 1 target. Hit: 17 (2d8 + 8) bludgeoning 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 20 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.

Breath Weapons (Recharges 5/6). The styx dragon uses one of the following breath weapons.

Acid sludge breath. The styx dragon exhales 3 globs of acidic sludge which can land in any three spaces they can see within 90ft of themselves. Each glob takes up a 10ft by 10ft space and if a creature starts their turn or ends their turn on one of those spaces they must make a DC 23 Constitution saving throw or take 49 (14d6) acid damage. These globs last for 3 rounds. On a successful saving throw a creature takes half damage. 

Stupefying breath. The styx dragon exhales a dark grey gas in a 90ft cone. Each creature within the radius must make a DC 23 Constitution saving throw or have the creature's intelligence score reduced by 4. 

The ancient styx 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 ancient styx dragon regains spent legendary actions at the start of its turn.

Detect (Costs 1 Action). The dragon makes a Wisdom (Perception) check.

Tail Attack (Costs 1 Action). The dragon makes a tail attack.

Water curse (Costs 3 Actions). The styx dragon speaks words of power in relation to the waters of the river styx. Each creature that is in water within 60ft of the styx dragon must make a DC 23 Wisdom saving throw or take 1d4 acid damage per 5ft of movement in that water within the 60ft radius of the dragon for the next 1d4 rounds.