Legendary Resistance (3/Day). If the Lizard fails a saving throw, it can choose to succeed instead.

Magic Resistance. The Lizard has advantage on saving throws against spells and other magical effects.

Reactive Immunity.  As a bonus action on each of its turns, the Lizard can gain one additional immunity to a damage type, which must be a type of damage it has suffered since the end of its last turn.  The Lizard can have no more than 3 additional immunities (not counting Bludgeoning, Piercing, and Slashing Damage from non-magical attacks) at one time.  If the lizard already has 3 immunities, it can instead use its bonus action to either end one immunity, or change one immunity to another damage type.  The Lizard cannot become immune to acid or necrotic damage using this ability.

Regeneration.  The Lizard regains 30 Hitpoints at the start of its turn.  If the Lizard takes acid or necrotic damage, this trait doesn't function at the start of The Lizards next turn.  The Lizard dies only if it starts its turn with 0 hitpoints and doesn't regenerate.

Resist Petrification.  If the Lizard is subjected to a spell or effect that causes petrification, such as the flesh to stone spell, the creature can choose to have the petrification effect take place immediately.  At the start of the lizards next turn.  The creature explodes, taking 55 (10d10) force damage, and all creatures within 30 feet of the lizard must make a DC 18 Dexterity Saving throw, taking 55 (10d10) piercing damage on a failed save, and half damage on a success.

Aggressive.  As a bonus action, the Lizard can move up to its speed towards a hostile creature it can see.

Multiattack. The Lizard can use its Frightful Presence or its Enlarge ability (if available). It then can make four attacks: one with its bite, two with its claws, and one with its tail. 

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the lizard can't bite another target.

Claw. Melee Weapon Attack: +15 to hit, reach 10ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 18 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the Lizards's choice within 120 feet of it and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Lizard is within line of sight, 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 the Lizard's Frightful Presence for the next 24 hours.

Enlarge (Recharge 5/6). The Lizard increases in size from Large to Huge for one minute.  While enlarged, the creatures movement speed is increased by 10 feet,  the reach of its attacks are increased by 5 feet, and damage rolls from its attacks are increased by one damage die.  If the Lizard takes 60 damage or more on a single turn from a creature, the Lizard must succeed on a DC 20 Constitution saving throw at the end of that turn or this effect ends early.  Rolls to recharge this ability do not start until the effect ends.

The Lizard 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. The Lizard regains spent legendary actions at the start of its turn.

Attack. The Lizard makes one claw attack or tail attack.

Move. The Lizard moves up to half its speed.