Ancient Green Dragon

Gargantuan Dragon, lawful evil

AC: 21 (natural armor)
HP: 402 (23d20 + 161)
Speed: 40 ft., fly 80 ft., swim 40 ft.

STR	DEX	CON	INT	WIS	CHA
27 (+8)	12 (+1)	25 (+7)	20 (+5)	17 (+3)	22 (+6)

Saves: Str +8, Dex +8, Con +7, Int +5, Wis +10, Cha +6
Skills: Deception +13, Perception +17, Persuasion +13, Stealth +8
Immunities: poison; poisoned
Senses: blindsight 60 ft., darkvision 120 ft., passive Perception 27
Languages: Common, Draconic
CR: 22 (XP 41,000)

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.

Actions

Multiattack.

The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Mind Spike (level 5 version).

Rend.

Melee Attack Roll: +15, reach 15 ft. 17 (2d8 + 8) Slashing damage plus 10 (3d6) Poison damage.

Poison Breath.

Constitution Saving Throw: DC 22, each creature in a 90-foot Cone. Failure: 77 (22d6) Poison 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 21):

At Will: Detect Magic, Mind Spike
1/Day Each: Geas, Modify Memory

Legendary Actions

Mind Invasion.

The dragon uses Spellcasting to cast Mind Spike (level 5 version).

Noxious Miasma.

Constitution Saving Throw: DC 21, each creature in a 30-foot-radius Sphere [Area of Effect]|XPHB|Sphere centered on a point the dragon can see within 90 feet. Failure: 17 (5d6) Poison damage, and the target takes a -2 penalty to AC 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.

Pounce.

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