Dhergoloths are the riot cops of Gehenna, fiends with a unique knack for mowing down mobs. Creatures of instinct, they show little independent judgment and no flexibility; if you start them up, they never stop.
Their exceptional Strength and Constitution stand out among their other, very unexceptional ability scores. They’re brutes, and they wouldn’t make ranged attacks even if they had any ability to do so. Their bodies are organic riot gear, resistant to physical damage from nonmagical weapons, cold, fire and lightning, and fully immune to acid and poison. They have 60 feet of darkvision and 60 feet of blindsight (I’m not sure why they have darkvision when they also have blindsight with the same range, which obviates the need for darkvision), which, as we’ll see, is key to their approach to combat.
Dhergoloths can cast darkness and fear at will. Darkness is often a problematic spell: since it thwarts darkvision, a character or creature that casts it inconveniences itself as well as its opponents, unless it has blindsight. But dhergoloths do have blindsight, so darkness works well for them indeed. Fear can also be problematic if you want to kill your foes rather than simply make them go away, and this is as true for dhergoloths as it is for most creatures. Given that both fear and darkness require concentration, and thus can’t be used at the same time, darkness is nearly always the better choice—and dhergoloths don’t have the Intelligence to recognize situational exceptions. They’ll cast fear only if ordered to, and not always then.
They also have three uses per day of sleep. By the time player characters are capable enough to fight a dhergoloth—we’re looking at level 5, minimum, and probably level 11 or higher if the dhergoloth is one of a group of foes—an innately cast sleep spell, which can’t be boosted and therefore affects only an average of 22 hp of creatures, isn’t likely to take down even a single PC. It might take down five commoners, but that’s the most you can reasonably hope for from it. This is disappointing, because if a dhergoloth were capable of putting any meaningful number of PCs to sleep, it would make a couple of scary attacking combinations possible.
Dhergoloths have two offensive actions, a simple Claw attack (its Multiattack lets it Claw twice) and Flailing Claws, which is on a recharge. Note well the wording of Flailing Claws, which includes, “The dhergoloth moves up to its walking speed in a straight line.” In other words, this is a Dash action that also deals damage, with the limitation that it can’t change direction once it starts moving. When it uses this action, it still has its entire regular movement in addition to the movement it gains from Flailing Claws.
Unlike a lot of damage-dealing actions that require saving throws to resist, Flailing Claws deals no damage on a successful save, only on a failure. Its DC of 14 isn’t terribly hard to beat: most mid- to high-level adventurers will be able to do so a little more than half the time. So when should a dhergoloth use it? Normally, per the Targets in Area of Effect table in chapter 8 of the Dungeon Master’s Guide, a 30-foot linear effect shouldn’t be expected to include more than one opponent, so anything more than that is dandy, right? Except that you also have to account for the likelihood that roughly two out of three targets will succeed on their saves. Therefore, I’d say a dhergoloth doesn’t use Flailing Claws unless it can plow through at least three opponents along its path; that way, it has a reasonable chance of carving up at least one of the three. If this requires some repositioning beforehand, it uses its movement to do so, and it doesn’t worry about opportunity attacks unless and until it’s been struck by a magic weapon.
Now, in the hypothetical scenario in which a dhergoloth could actually make sleep work, the unconscious victims would automatically fail their Dex saves, and Flailing Claws would hit all of them. But sleep, unboosted, has a hard ceiling of 40 hp; by level 11, even most wizards will have that, while barbarians will often have that many hit points by level 4. Unless you’re intentionally going for a shamefully unfair TPK against a party of low-level patsies, sleep is only good for use on innocent bystanders.
Wait—did I say “a 30-foot linear effect” up there? That’s only true if the dhergoloth’s base speed is unmodified. If the dhergoloth is subjected to an effect that increases its speed—say, a haste spell, which doubles it—that effect increases the range of Flailing Claws by the same amount.
When Flailing Claws is on cooldown, the Claw/Claw Multiattack is the default. This is where darkness comes into play, because being inside the Sphere of Unholy Murk grants the dhergoloth advantage on its attack rolls and imposes disadvantage on those of its opponents, unless they have true seeing or some other means of seeing through magical darkness (and that possibility is too advanced for a dhergoloth to contemplate). Once cast, darkness can’t be moved—it could be if a dhergoloth cast it on an object, but alas, they’re not smart enough to try that—so it’s more helpful in confined spaces than in large, open ones.
Darkness takes an action to cast, can be sustained for up to 10 minutes and requires concentration. This suggests not only that a dhergoloth casts it immediately at the beginning of combat but that it probably does so before combat even begins, to exploit its targets’ confusion. Like, if this thing is coming at you, and suddenly darkness descends, you have a good idea what’s happening. But if you’re just going about your business and suddenly everything in sight is plunged into unnatural darkness, you have no way of knowing what’s coming next. Anytime darkness is knocked down—e.g., with a daylight spell, or by breaking their concentration—their next action is to cast it again.
In addition to their various damage immunities and resistances, dhergoloths have quite a few hit points. For a hit to register as painful, it has to deal at least 12 damage after any applicable reduction. When this happens, a dhergoloth uses Flailing Claws if it’s available—but if it’s not, rather than Multiattack, it Teleports to another location, ideally another location within its sphere of darkness. (Teleport takes it to “an unoccupied space it can see”; blindsight allows it to “see” through darkness, well enough for it to count for the purposes of the feature.) When a dhergoloth uses Flaming Claws after a hit that deals 12 or more damage, it doesn’t worry about how many opponents it can get; its top priority is to retaliate against the enemy who hurt it, and collateral damage is nice but not necessary. If engaged in melee by two or more foes who can inflict damage that it’s not immune or resistant to, it Teleports away—preferably next to a weaker foe whom it can abuse with impunity.
When dhergoloths aren’t busily engaged in melee, they aggressively pursue foes who try to run away. If their movement alone won’t allow them to keep up, they’ll use the Teleport action to jump into a foe’s path—within their 5-foot reach, so that if the foe tries to turn and run back the way they came, they’ll be subject to an opportunity attack.
Like all yugoloths, a dhergoloth destroyed outside the plane of Gehenna isn’t killed, only driven back to that plane to re-form. However, dhergoloths aren’t bright enough to think about the implications of what plane they’re on (or, possibly, even to know what plane they’re on), and their normal self-preservation instinct kicks in when they’re seriously wounded (reduced to 47 hp or fewer), whether they’re in Gehenna or not. That instinct moves them to Teleport 60 feet away—the maximum possible distance—and keep going with the remainder of their movement.