I’ve been asked to take a look at mephits, wicked little critters that maliciously embody the para-elements of dust, ice, magma, mud, smoke and steam. The Monster Manual characterizes them as “tricksters,” but every one of them is of neutral evil, not chaotic, alignment, so their “trickery” is of a decidedly baleful sort. I don’t see any reason why they shouldn’t behave as evolved creatures with respect to their self-preservation instinct, but if survival is their No. 1 priority, causing gratuitous harm and annoyance to others is No. 2.
Mephits aren’t tough—half of them are CR 1/4, and the other half are CR 1/2. All of them have low Strength, all of them can fly, and all of them have darkvision (meaning they either live underground or are active primarily at night) and the Death Burst feature, which does something when they’re killed, although that something depends on the type of mephit. And they all have a simple melee attack, along with a breath weapon that has only a 1 in 6 chance to recharge, so in all likelihood, they’ll get to use it only once. Most (but not all) of them are proficient in Stealth, suggesting that they like to ambush their victims, and their low Strength suggests that they’ll usually be encountered in decent-size groups; a lone mephit wouldn’t dare pick a fight with more than a couple of enemies at once.
Beyond that, though, every type of mephit is a little bit different, and there’s nothing for it but to look at each type individually.
Dust mephits have high Dexterity and average Constitution, indicating a preference for ranged combat. They’re vulnerable to fire, so they’ll keep their distance from torches and campfires, and the second an opponent casts a spell that does fire damage or brandishes a flaming weapon, they’ll hightail it.
A dust mephit leads with its Blinding Breath, attacking from hiding so as to gain surprise and waiting until it can catch at least two opponents in its 15-foot cone of effect. (If it can get more, so much the better, but if it can get two, it won’t delay in order to see whether it can get three.)
What it does on its second round of combat depends on how much luck it had in the first. It will engage a blinded enemy with its claws, but not if there’s an unblinded enemy who can intervene. If it can use its Blinding Breath again, it will, but in most cases, it can’t. Without an ally to back it up, it will simply fly away, using the Dash action to double its movement and satisfying itself with the momentary debilitation of its victim(s). If every enemy is blinded, on the other hand, it will seize the advantage and attack one with its claws. It’s indiscriminate in its selection of targets; it slashes at whoever’s closest.
As soon as an enemy shakes off its dust blindness, unless it can use its Blinding Breath again, the dust mephit takes off. It’s not bright enough to Disengage and not nimble enough to Dodge as it flees; it will simply Dash. It also flees if its blinded opponent, despite having disadvantage, manages to land a lucky blow and reduce the dust mephit to 6 hp or fewer. Before it does, though, it drops its once-per-day use of sleep, in the hope of knocking out its enemy before he or she can make an opportunity attack against the fleeing mephit.
Ice mephits are similar to dust mephits, except that they’re also vulnerable to bludgeoning damage, so they’ll stay well clear of anyone carrying a blunt weapon. Other than that, their tactics are similar to those of dust mephits. They lead with Frost Breath, again trying to catch at least two enemies in the chill cone. However, since Frost Breath doesn’t grant advantage to ice mephits or impose disadvantage on their opponents, they don’t follow it up with melee attacks. Instead, they fly around out of reach, waiting for their Frost Breath to recharge, and Readying the Dash action in case an opponent comes within 10 feet so that they can avoid melee engagement. This assumes they’re being chased by melee opponents; if they’re not, but they’re being shot at by a ranged attacker, they Dodge instead. Dealing with both is too much for them. In that instance, they flee.
If an ice mephit takes serious damage (reduced to 8 hp or fewer), it drops its once-per-day fog cloud on an area that includes its attacker, then exploits its enemy’s blindness to make its escape, since a blinded melee opponent will have disadvantage on its opportunity attack. Other, less injured mephits won’t fly into the fog in order to attack the enshrouded enemy, because they’d be blinded, too: darkvision doesn’t help a creature see in an area heavily obscured by something other than darkness.
Magma mephits have a slightly above-average Constitution to go with their slightly above-average Dexterity, making them skirmishers, more willing than dust or ice mephits to fight up close. Also unlike those two, magma mephits lead with their once-per-day innate spell, heat metal, because it has the potential both to enhance their action economy and to impose disadvantage on the enemy’s attacks. Each magma mephit in the encounter picks an enemy wearing metal armor or wielding a metal weapon and casts heat metal to make it glow red-hot. On subsequent turns, if that enemy is still wearing the armor or holding the weapon, it uses its bonus action to deal this damage again.
While they have fewer qualms about engaging in melee than dust or ice mephits do, they do like to have the upper hand, so they engage in melee with their claws only against opponents who either have dropped their weapons, are clinging grimly to red-hot weapons that are burning them or are wearing armor that’s causing them similar discomfort. Against opponents who aren’t wearing metal armor and are holding nonmetal weapons, they use their Fire Breath instead, positioning themselves out of immediate melee reach and, if possible, where they can catch at least one other enemy in the cone of effect. Until their Fire Breath recharges, they do their best to stay out of reach of melee attacks, Readying a Dash action in case any opponent comes within 10 feet, or Dodging if they have to avoid ranged attackers but not melee attackers.
Once an opponent has retrieved his or her no-longer-hot weapon, the magma mephit is done, as it is if it’s seriously wounded (reduced to 8 hp or fewer), and it flies away, using the Dash action.
Mud mephits, like magma mephits, are skirmishers. Unlike any of the mephits mentioned so far, they have no innate spellcasting ability they can use to inconvenience enemies or cover their escape. What they can do, though, is restrain enemies with their Mud Breath (I’m not sure how mud can be exhaled, exactly; I feel like this must be more of a mud belch or a mud vomit), and this is the feature they lead with.
Like dust mephits, if mud mephits’ “breath” weapon doesn’t restrain all their opponents, they’ve got basically nothing, and they have to submerge themselves and scram. (Mud mephits can swim, presumably through mud as well as water, so this, rather than flying, is their best mode of departure.) If they do manage to restrain all their opponents, it’s clobberin’ time, starting with the nearest restrained enemy.
If an enemy breaks free of his or her restraint, the mud mephit beating on it isn’t fast enough to run away or even swim away; it’s pretty much got to resign itself to its fate. Notice that while the preceding three types of mephits are all CR 1/2, mud mephits are CR 1/4, and because of this, I think they’ll have to be present in even larger numbers—at least three of them for every two enemies, preferably more—before they’ll bother picking a fight in the first place.
Smoke mephits are also CR 1/4, so again, they won’t start a fight unless there are at least three of them for every two enemies. They do have innate spellcasting, but the spell they can cast once per day, dancing lights, is useless in combat. I’d say they use it only to mess with people they don’t want to actually fight but feel like harassing anyway.
If and when they do decide to start a fight, they lead with Cinder Breath, which, like dust mephits’ Blinding Breath, can blind opponents in its cone of effect. In all other respects (minus the sleep spell, which they don’t have), smoke mephits follow the same modus operandi as dust mephits.
Steam mephits are CR 1/4, but they’re a little better equipped to hold up once a fight starts than mud and smoke mephits are. They lead with blur, which they cast innately, before revealing themselves to their victims, and only then let loose with their Steam Breath against their nearest foes—as always, trying to include at least two enemies in the cone of effect.
Because blur gives their opponents disadvantage on attacks against them, they have less to fear from opportunity attacks, and so, as skirmishers, they use the tried-and-true flier tactic of stationing themselves up in the air, flying down to attack with their claws, then flying back up, out of reach of melee attackers. This incurs an opportunity attack each time they leave their opponents’ reach, but as mentioned, this attack is made with disadvantage, and an opponent with Extra Attack doesn’t get that additional swing when attacking as a reaction. If their Steam Breath recharges, they’re more than happy to use it again, flying down just low enough to do so.
Steam mephits flee when seriously wounded (reduced to 8 hp or fewer), using the Dash action to fly away.