Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
Liquid Metal. The dragon can move through a space as narrow as 1 inch wide without squeezing.
Mercurial Splash. When the dragon takes damage of any type but psychic, each creature within 5 feet of the dragon takes 5 (1d8) acid damage.
Melting Point. When the dragon takes 20 or more cold damage in a single round, its Liquid Metal trait ceases to function. When the dragon takes 20 or more fire damage, its Mercurial Splash trait instead does 10 (2d8) acid damage. If the dragon takes 20 or more cold and 20 or more fire damage in a single round (or less than 20 of either), both these features remain unchanged.
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: +12 to hit, reach 10 ft., one target. Hit: 17(2d10 + 6) piercing damage and 3(1d6) acid damage.
Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 13(2d6 + 6) slashing damage and 3(1d6) acid damage.
Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 15(2d8 + 6) bludgeoning damage and 3(1d6) acid 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 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, 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.
Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.
Mercurial Breath (Recharge 5–6). The dragon exhales liquid mercury in a 60-foot cone. Each creature in that area must make a DC 19 Dexterity saving throw, taking 56 (16d6) acid damage on a failed save, or half as much damage on a successful one.
Slowing Breath. The dragon exhales gas in a 60-foot cone. Each creature in that area must succeed on a DC 19 Constitution saving throw. On a failed save, the creature can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the creature can use either an action or a bonus action on its turn, but not both. These effects last for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself with a successful save.
The 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 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 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Lair and Lair Actions
A Mercury Dragon’s Lair
Mercury Dragons lair themselves away from prying eyes, in the coldest reaches of the world. In these glacial tundras and frigid wastelands, they dig expansive tunnels below the surface, lairing only when they find a natural source of heat. In these labyrinthine expanses, the Mercury dragons dwells; solid in the cold and nearly liquid in the heat.
Lair Actions
On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects:
- The dragon chooses a point on the ground that it can see within 120 feet of it. Toxic Mercurial vapour seeps from the ground in a 20-foot radius centered on that point. A creature inside that sphere when it appears must make a DC 15 Constitution saving throw, taking 2d10 poison damage and becoming poisoned on a failed save. A poisoned creature can repeat this saving throw at the end of each of its turns, ending the effect on a success.
- The dragon chooses a 10-foot-square area on the ground that it can see within 120 feet of it. The ground in that area turns into 3-foot-deep liquid mercury. Each creature on the ground in that area when the mud appears must succeed on a DC 15 Dexterity saving throw or sink into the liquid metal and become restrained. A creature can take an action to attempt a DC 15 Strength check, freeing itself or another creature within its reach and ending the restrained condition on a success. Moving 1 foot in the mud costs 2 feet of movement. On initiative count 20 on the next round, the mud hardens, and the Strength DC to work free increases to 20.
Regional Effects
The region containing a legendary mercury dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:
- Water within 6 miles of the dragon's lair is toxic and takes on a slight silvery sheen. Any creature who attempts to drink from it must make a DC 15 constitution saving throw or take 1d6 poison damage and become poisoned for a minute.
- It is unnaturally cold within 6 miles of the dragon's lair. The DC for exhaustion based on cold weather (if applicable) increases by 3.
- The air within 6 miles of the dragon's lair is corrosive. Any non-magical objects composed of iron, steel, aluminum or silver quickly rust and tarnish. Any item of these materials kept within 6 miles of the lair for longer than 72 hours is destroyed.
If the dragon dies, the toxic water fades over the course of 1d10 days. The other effects end immediately.
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