The “four elements” of air, earth, fire and water originated with the Greeks, but somewhere along the line, some Dungeons and Dragons writer must have read that jinn, in Arab myth, were supernatural beings of air and that efreets were supernatural beings of fire; decided that there had to be corresponding water and earth spirits, too; and shoehorned marids into the genies-of-water role, maybe because of the syllable mar-, which means “sea” in Latin. In Arabic, however, مارد mārid means “defiant” or “rebellious,” and it’s used to describe all sorts of troublemaking creatures, including not only certain genies but demons and giants as well.
The D&D marid, like its fiery cousin, the efreet, is a brute fighter with extraordinarily high Strength and Constitution but also extraordinarily high mental attributes. Like jinn, marids have proficiency in Dexterity, Wisdom and Charisma saving throws along with a Constitution high enough to make saving throws easily without proficiency, so they’ll have little to fear from spellcasters.
The marid’s equivalent of a jinni’s Create Whirlwind and an efreet’s Hurl Flame is Water Jet, a linear, guaranteed-damage attack that can push enemies away and knock them prone. Based on this feature’s 60-foot range, there’s not much reason to expect it to affect more than two creatures at once (based on the “Targets in Areas of Effect” table on page 249 of the Dungeon Master’s Guide), and since the marid can always position itself to line up any two opponents in its sights, is there any reason for it not to use this feature again and again?
Since Water Jet consumes a full action, and since anyone can “clear” the prone condition on its own turn simply by spending
5 feet of movement half of one’s movement speed to get up, there’s not much opportunity for a combo here unless the marid is accompanied by other creatures. On the other hand, even disregarding knockdown, a Water Jet aimed at two targets can be counted on to do more damage, on average, than two trident strikes as long as the Water Jet targets’ Dexterity saving throw modifiers are less than 10(AC – 6.5) / 7, where AC is the trident target’s armor class. Yes, I did the stupid, ugly math. It was stupid and ugly. And it produces only a handful of plausible cases in which two trident attacks are expected to do more damage, and in all of these, the Dex save bonus is +8 or greater (+3 or greater with advantage on the save, such as when a barbarian is using the Danger Sense class feature) and the AC is 12 or less, so you don’t even need to look at cases beyond these. Marids have Intelligence 18. They know, intuitively, which attack will do more damage against which targets. And that’s Water Jet, unless its prospective targets are exceptionally nimble, while those it can poke with a trident are poorly armored.
Water Jet also makes a good “damaging Disengage,” if the marid wants to reduce its risk of incurring an opportunity attack while repositioning itself. It’s good for exploiting the environment by knocking a PC into a hazard, over a precipice or off the deck of a boat. And if the marid is accompanied by one or more minions, they can increase their expected damage by attacking a prone character with advantage.
But there is one thing that may boost the appeal of a trident attack. I’ll get to it in a second.
The marid has the same standard “genie package” of gaseous form, invisibility, plane shift and conjure elemental; unlike the jinni and the efreet, it lacks major image. It has a few other tricks up its sleeve, though.
Control water is a powerful once-per-day ability that allows the marid to create a whirlpool in a body of water which can catch a PC in its vortex and subject him or her to bludgeoning damage; to flood an area with any amount of standing water; or to create a 20-foot-high tidal wave and bring it crashing down on its enemies.
But not to be underestimated is fog cloud, which the marid can cast at will. It does require concentration, so it can’t be cast at the same time as control water, conjure elemental or invisibility. But what it can do is heavily obscure an area, blinding the PCs but not bothering the marid at all, since it has blindsight out to a distance of 30 feet. Within that radius, the marid will have advantage on melee attacks against blinded PCs, while they’ll have disadvantage on attacks against it, and a barbarian blinded by fog can’t use Danger Sense to dodge a Water Jet; meanwhile, ranged attackers and spellcasters outside the fog cloud won’t be able to make out anything inside it. With the advantage provided by fog cloud, trident attacks become feasible against moderately armored opponents (up to AC 15), but only if the prospective targets of a Water Jet have the same high Dex save bonuses as mentioned above.
The marid doesn’t need to cast water breathing or water walk: it’s amphibious. But it can cast these on PCs if they can persuade it to do them a favor (the same goes for create or destroy water and purify food and drink). That will take some doing, however. Chaotic neutral by alignment, marids are haughty and whimsical; their desires change by the moment, and they’re quick to take offense.
Generally speaking, therefore, a marid will rely on Water Jet as its default damage-dealing method, casting conjure elemental to take advantage of PCs’ being knocked prone. If the PCs are dodging Water Jet with ridiculous ease, the marid will drop a fog cloud on itself and start stabbing poorly and moderately armored opponents with its trident. If it’s in the water, and the PCs are also in it or on it, it may use control water to create a whirlpool or jumbo wave. And if it takes an extra-special dislike to a particular PC, it may try to plane shift him or her to the Elemental Plane of Water.
If the PCs can reduce it to 160 hp or fewer, or if they can reliably deal it 17 hp of damage or more per turn, it will look to strike a bargain, although what it perceives as its interests may strike the PCs as quite eccentric or offensive. If seriously injured (reduced to 91 hp or fewer), it will skedaddle, using gaseous form, invisibility or plane shift.