Ancient Yellow Dragon

Amphibious. The dragon can breathe air and water.

Legendary Resistance (4/Day, or 5 in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes three Rend attacks. 

Rend. Melee Attack Roll: +14, reach 15 ft. Hit: 25 (4d8 + 7) Slashing damage plus 14 (4d6) Force damage. 

Salty Breath (Recharge 5–6). Dexterity Saving Throw: DC 21, each creature in a 90-foot Cone. Failure: 56 (16d6) Force damage, and the target's speed is Restrained by solid salt. A creature can take an action to end the Restrained condition on the target. Success: Half damage only.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 18, +10 to hit with spell attacks):

At Will: Control Water, Fog Cloud (level 7 version)

1/Day: Cloudkill (level 7 version), Control Weather, Whirlwind

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 its turns.

Time Shaper. The dragon uses Spellcasting to cast Slow. The dragon can’t take this action again until the start of its next turn.

Tide Shaper. The dragon uses Spellcasting to cast or manipulate [spells]Fog Cloud[spells] (level 5 version). The dragon can’t take this action again until the start of its next turn.

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