Ancient Abyssal Dragon (Fomirza)

Amphibious. Fomirza can breathe air and water.

Demonic Heritage. Fomirza counts as Dragon and Fiend for any spell-like ability or effect.

Legendary Resistance (3/Day). If Fomirza fails a saving throw, she can choose to succeed instead.

Living Shadow. While in dim light or darkness, Fomirza has resistance to damage that isn't force or radiant. 

Shadow Stealth. While in dim light or darkness, Fomirza can take the Hide action as a bonus action.

Innate Spellcasting. Fomirza's innate spellcasting ability is Charisma (spell save DC 25). She can innately cast the following spells, requiring no material components:

1/day each (at 8th level): Counterspell, Disintegrate, Heal, Illusory Dragon, Mind Blank, Maddening Darkness, Power Word Stun, Shield, Summon Greater Demon (CR 9 or lower)

Eldritch Claws. Fomirza's unarmed strikes are magical.

Multiattack. Fomirza can use its Frightful Presence. She then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage plus 11 (3d6) necrotic damage.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Frightful Presence. Each creature of Formirza's choice that is within 120 feet of Formirza and aware of it must succeed on a DC 25 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 Formirza's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). Formirza uses one of the following breath weapons.

Maddening Breath. Formirza exhales maddening energy in a 90-foot cone. Each creature in that area must make a DC 25 Wisdom saving throw, taking 59 (13d8) psychic damage on a failed save, or half as much damage on a successful one. A humanoid reduced to 0 hit points by this damage dies becomes a Vampiric Mind Flayer with 50% of its total heath and acts immediately after Formirza in the initiative count. The undead mind flayer is under the Formirza's control.

Paralyzing Breath. Formirza exhales gas in a 90-foot cone. Each creature in that area must succeed on a DC 25 Constitution saving throw or be paralyzed 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.

Change Shape. Formirza magically polymorphs into a humanoid or beast that has a challenge rating no higher than her own, or back into her true form. She reverts to her true form if she dies. Any equipment she is wearing or carrying is absorbed or borne by the new form (Formirza's choice).

In a new form, Formirza retains her alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Her statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Adamantine Hide (Recharge 5–6). In response to a critical hit, Fomirza can choose to gain immunity to critical hits, to treat this and all further critical hits as normal hits. This effect lasts until the start of Formirza's next turn.

Fomirza 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. Fomirza regains spent legendary actions at the start of her turn.

Detect. Fomirza makes a Wisdom (Perception) check.

Tail Attack. Fomirza makes a tail attack.

Wing Attack (Costs 2 Actions). Fomirza beats her wings. Each creature within 15 feet of Formirza must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Fomirza can then fly up to half her flying speed.

Summon Undead (Costs 3 Actions). Up to 1d4 Deathlocks appear in unoccupied spaces within 40 feet of Fomirza and remain until destroyed.  Undead summoned in this way roll initiative and act in the next available turn.  Fomirza can only have 4 Deathlocks  summoned by this ability at a time.