Gargantuan Dragon (Chromatic), Chaotic Evil
Traits
Amphibious. The dragon can breathe air and water.
Legendary Resistance (4/Day, or 5/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.
Spellcasting. The 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)
Actions
Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Melf's Acid Arrow (level 4 version).
Rend. Melee Attack Roll: +15, reach 15 ft. Hit: 17 (2d8 + 8) Slashing damage plus 9 (2d8) Acid damage.
Acid Breath (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.
Legendary Actions (3)
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 21, one creature the 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 dragon can't take this action again until the start of its next turn.
Frightful Presence. The dragon uses Spellcasting to cast Fear. 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.
Found on p.40 of MM2025 (Free Rules)