The lowest-level of the yugoloths in Mordenkainen’s Tome of Foes is the merrenoloth, and it’s something I haven’t come across before: a low-level monster that nevertheless has lair actions with regional effects. Merrenoloths normally pilot ferryboats on the River Styx (meaning, I suppose, that Charon is the head of their guild), but they can be summoned and hired to captain vessels on other planes. When a merrenoloth accepts such a contract, the vessel becomes its lair.
The lair actions don’t do much to protect the merrenoloth itself; rather, they protect the vessel under its command, allowing the merrenoloth to restore hit points to a damaged vessel, speed it away from pursuers (or toward another vessel it’s pursuing) or interfere with flying attackers. The merrenoloth itself, in fact, would rather not fight at all—it would rather just do its job. Consequently, it will never come to the aid of another creature. It engages in combat only if threatened directly.
When it is threatened, it defends itself partly by spellcasting and partly by striking with an oversize oar, which it seems to wield as a finesse weapon, judging by its attack bonus and damage. Its ability contour is interesting: part spellslinger, part melee shock attacker. But it doesn’t have much in the way of damaging ranged spells; its strongest offensive gambit is gust of wind, which it can use to try to shove enemies overboard. I say “try to,” because its spell save DC of 13 is not impressive; even low-level fighting classes will beat it at least half the time. On the other hand, it affect an area 60 feet long and 10 feet wide, so it’s usable against several foes at once, at least one of whom is likely to fail their save.
Without strong spells to lob from a distance, the merrenoloth has to find a combat sequence that allows it to do a bunch of damage, then skedaddle. Its Teleport trait helps with this, allowing it to bamf 60 feet in any direction as a bonus action. This addition to its action economy is crucial, granting it extra mobility to accompany its attack. How much? Enough to carry it to almost any location on the deck of the schooner depicted in Appendix C of the Dungeon Master’s Guide. (According to the naval combat rules in the Unearthed Arcana article “Of Ships and the Sea,” this schooner would be a Sailing Ship with two ballistae and no mangonel.)
The merrenoloth’s first priority is to protect its vessel, and its second is to protect itself; there is no third priority. To protect its vessel from boarders, above all else, it must maintain control over the helm and the mainmast. Therefore, its first action in combat is to cast darkness at the fore edge of the quarterdeck, engulfing most of the rear half of the ship in blackness. Darkness is usually a double-edged sword, since darkvision doesn’t penetrate it—but the merrenoloth has blindsight, suckas!
To complete its turn, it uses Teleport as a bonus action: If it’s outside the sphere of darkness, it teleports inside, and if it’s already inside the sphere, it transports itself 10 feet from its current position to some other point inside it, so that anyone trying to attack the square they last saw it in will connect with nothing but air.
During combat, for the most part, the merrenoloth is content to defend itself. It targets those who’ve attacked it, leaving others be. As much as possible, it does its fighting within the sphere of darkness; if its concentration on that spell is broken somehow, it casts it again as soon as it can. Otherwise, it uses its Oar attack against a melee opponent within reach (with advantage, natch, since it can sense its opponent but its opponent can’t see it), then its Fear Gaze, which it uses first against melee foes and then against anyone attacking it at range. Finally, as a bonus action, it Teleports again to another point in the sphere, just to mess its opponents up.
Darkness, incidentally, doesn’t shut down its ability to use Fear Gaze—its blindsight allows it to “see” its targets without actually seeing them. (That’s the official word from Mike Mearls.) However, being frightened of the merrenoloth in the area of a darkness spell is a funny thing. The darkness itself imposes disadvantage on the frightened foe’s attack rolls, but being frightened doesn’t, because the foe can’t see the merrenoloth! However, they still can’t willingly move closer to the source of their fear, meaning that they may want to move in a certain direction yet find themself mysteriously unable to—because the merrenoloth is that way, even though the foe can’t see it. Weird, huh? The upshot is, Fear Gaze is primarily prophylaxis: a way to keep imposing disadvantage on incoming attacks even if darkness is dispelled.
The Oar/Fear Gaze Multiattack, combined with Teleport, is solid, but what the merrenoloth really wants is to perform its pièce de résistance combo, which begins with gust of wind. All the time it’s fighting, the merrenoloth is watching carefully for the chance to blow at least four of its enemies down (along with no more than one or two of the crew of its own vessel). When they’re fortuitously arrayed, the merrenoloth gets into position (via normal movement if it can, Teleport if it must) and casts gust of wind to knock as many of them overboard as it can. Most likely, one or two foes will stand firm, and the merrenoloth will just have to continue smacking them with its oar. But if it gets lucky and manages to blow four or more opponents into the water, it follows up on its next turn with control water, creating a Whirlpool beneath them. Then, if it’s managed to clear the deck of all its enemies, it uses its strong wind lair action to sail away. Bon voyage!
A merrenoloth’s ship never sinks, “even if its hull is breached.” But if the ship is completely smashed to smithereens and the merrenoloth falls in the water—well, for one thing, it’s released from its contract, because there’s no longer any ship to captain. It also has a 40-foot swimming speed, superior to its walking speed, which allows it to flee effectively from any pursuers. As it flees, it uses Fear Gaze to further dissuade any especially good swimmers before switching to the Dash action. As long as its ship remains seaworthy, however, it fights to the death.
That being said, a merrenoloth can tell very quickly if a fight is hopeless, and since its chief obligation is to guide its vessel safely to its destination, it will promptly call a halt to combat and offer to parley if there’s any chance at all that it can bargain or buy its way out of the situation rather than fight its way out.