Apophis

Amphibious. Apophis can breathe air and water.

Legendary Resistance (3/Day). If Apophis fails a saving throw, she can choose to succeed instead. When she does, any areas of magical Darkness in the lair are dispelled, and she can’t use Enshroud or Thwart Healing until the end of her next turn.

Shadow Strength. Acid and Necrotic damage dealt by Apophis ignore damage Resistance.

Multiattack. Apophis makes one Bite attack and two Claw attacks.

Bite. Melee Attack Roll: +15, reach 15 ft. Hit: 19 (2d10 + 8) Piercing damage plus 9 (2d8) Necrotic damage. If the target is a light source or is wearing or holding a light source, that light is extinguished.

Claw. Melee Attack Roll: +15, reach 10 ft. Hit: 15 (2d6 + 8) Slashing damage, and Apophis can move the target up to 15 feet horizontally.

Void Breath (Recharge 5–6). Dexterity Saving Throw: DC 22, each creature in a 90-foot-long, 20-foot-wide Line. Failure: 24 (7d6) Acid damage plus 24 (7d6) Necrotic damage. Success: Half damage. Failure or Success: The area is filled with magical Darkness for 1 minute. A creature with Darkvision can’t see through this darkness, and no light except a Daylight spell or a light-creating spell of 5th level or higher can illuminate it.

Enshroud. Dexterity Saving Throw: DC 19, one creature within 120 feet of Apophis. Failure: The target is Blinded and has Vulnerability to Necrotic damage until the start of Apophis' next turn.

Thwart Healing. Trigger: A creature within 60 feet of Apophis regains Hit Points. Response -- Constitution Saving Throw: DC 19, the triggering creature. Failure:  The creature regains half the number of Hit Points instead.

Apophis has three Villain Actions. She can take each action once during an encounter after an enemy’s turn. She can take these actions in any order but can use only one per round.

Burning Desire. Dexterity Saving Throw: DC 19, each creature within 90 feet of Apophis. Failure: An acid globule attaches to the target's body. A creature attached to a globule takes 14 (4d6) Acid damage at the start of its turns. A creature can use an action to remove a globule from itself or a creature it can reach.

Shadow Form. Apophis becomes semiincorporeal, gaining resistance to Bludgeoning, Piercing, and Slashing damage until the end of her next turn. She then teleports up to 120 feet to an unoccupied space that she can see.

Sinking Gloom. Apophis summons sticky black acid centered on a point she can see on the ground within 120 feet of herself, creating a 5-foot-deep, 20-foot-radius pool. The pool is Difficult Terrain and lasts for 1 minute. It causes either of the following effects:

Strength Saving Throw: DC 19, each enemy creature who starts its turn in the pool. Failure: The target is Restrained until the start of its next turn. Failure or Success: 24 (7d6) Acid damage.

Dexterity Saving Throw: DC 19, each enemy who starts its turn flying within 30 feet of the pool. Failure: 24 (7d6) Acid damage, and the target is pulled down by living acid, landing in the nearest unoccupied space of its choice within the pool. The target is also Restrained until the start of its next turn.