Adult mercury dragon

Mercury Form. The dragon can enter a form of liquid mercury as an action that can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing and lasts until the start of the mercury dragons next turn and in this form gains resistance to bludgeoning piercing and slashing from non magical weapons. 

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 15 (2d10 + 5) [piercing] damage plus 6 (2d6) poison damage. 

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 11 (2d6 + 5) [slashing] damage. 

Tail Melee Weapon Attack: +10 to hit, reach 15 ft., 1 target. Hit: 13 (2d8 + 5) [bludgeoning] damage. 

Frightful Presence. Each creature of the dragon’s choice that is within 120 feet of the dragon and aware of it must succeed on a DC 15 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 the dragon’s Frightful Presence for the next 24 hours .

Poison Breath (Recharge 5-6). The dragon exhales poisonous gas in a 60-foot cone. Each creature in that area must make a DC 15 Constitution saving throw, taking 56 (16d6) poison damage on a failed save, or half as much damage on a successful one.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon’s choice).

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

The adult mercury dragon 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 adult mercury dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 ft. of the dragon must succeed on a DC 15 Dexterity saving throw or take 11 (2d6 + 5) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.