Mercury Elementals

Hello, everyone, and Happy Friday! Today’s article provides a stat block for a new type of Elemental, the Mercury Elemental. This creature was interesting to design as, much like the Primordial Cultist, I wound up needing to design a creature with spellcasting abilities. Typically I try to avoid designing such creatures in my campaigns, as the ability to cast spells has a much harder measured effect on a monster’s Challenge Rating and adds difficulty for the Storyteller who has to cross-reference spells with their monster’s stat blocks. Despite these considerations, though, I felt that the ability to cast spells was important thematically to these creatures, and hope that you’ll find that you agree that without spells these enemy stat blocks would not accurately reflect their concepts in combat.

Mercury Elementals

Unlike other Elementals, Mercury Elementals possess a keen, alien intelligence which they use to manipulate followers of The Armored Tyrant to do its bidding. With bodies made of liquid metal, covered in pulsating arcane runes, Mercury Elementals are as dangerous to encounter off the battlefield as on it. Vapors from their body cause a victim to develop erethism, or mercury poisoning, rapidly, and one of these elementals would sooner be inclined to use its ability to cast Modify Memory to make a victim forget about their encounter. When the victim begins to act eccentrically around their peers and is shunned for their strange behavior, the Mercury Elemental will no longer have fear of being discovered in its covert operations.

Mercury Elemental

Large elemental, chaotic evil

  • Armor Class 19 (natural armor)
  • Hit Points 99(18d10)
  • Speed 30 ft., fly 30 ft.

5 (-3) 10 (+0) 10(+0) 20 (+5) 10 (+0) 13 (+1)

  • Damage Vulnerabilities force
  • Damage Resistances bludgeoning, piercing, and slashing from nonmagical weapons
  • Damage Immunities poison
  • Condition Immunities charmed, exhaustion, paralyzed, petrified, poisoned, unconscious
  • Senses darkvision 60 ft, passive Perception 10
  • Languages Common, Primordial, and any three other languages
  • Challenge 8 (3900 XP)

Liquid Form. The elemental can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Temperature Reactivity. If the elemental takes Cold damage, the elemental compacts into a smaller form as though under the effect of a Reduce spell for one minute. If the elemental takes Fire damage, the elemental expands into a larger form as though under the effect of an Enlarge spell for one minute. If the elemental takes both Cold and Fire damage in a single round, it has Disadvantage to Attacks and Saving Throws until the end of its next turn.

Spellcasting. The Elemental is a 9th-level spellcaster. Its spellcasting ability is Intelligence(spell save DC 16, +8 to hit with spell attacks). The elemental has the following wizard spells prepared:

Cantrips (at will): Magnetic Claws, Prestidigitation, Shocking Grasp

1st level (4 slots): Comprehend Languages, Mage Armor, Sleep

2nd level (3 slots): Cloud of Daggers, Heat Metal, Hold Person

3rd level (3 slots): Bestow Curse, Clairvoyance, Counterspell

4th level (3 slots): Confusion, Conjure Minor Elementals

5th level (1 slot): Modify Memory


Multiattack. The elemental makes two slam attacks.

Slam. Melee Weapon Attack: +3 to hit, reach 5ft., one target. Hit 9 (2d8) bludgeoning damage.


A creature who spends more than a minute within 30 feet of a Mercury Elemental with half or fewer of its Hit Points must attempt a DC 16 Constitution save. On a failure, the creature becomes infected with Erethism.

Erethism is a Disease which manifests through behavioral changes. An infected creature may at first become short-tempered, self-doubting, or generally apathetic. These symptoms begin to show 1d6 days after infection.

When a creature begins a Long Rest, if the creature had to speak or otherwise interact with any other intelligent creatures since its last Long Rest, the creature must succeed on a DC 16 Charisma saving throw or gain a level of Exhaustion on a failed save. On a successful save, the creature’s exhaustion level decreases by one level. If a successful saving throw reduces the infected creature’s level of exhaustion below 1, the creature has advantage on the next Charisma saving throw it makes as a result of erethism.

Erethism must be cured by a DC 18 Wisdom (Medicine) check or by magical means, such as a Paladin’s Lay on Hands or a Lesser Restoration.

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