A long while back, a reader asked me to look at the derro, a creature featured in Out of the Abyss. I didn’t have Out of the Abyss (and still don’t), so I had to table the request. But since Mordenkainen’s Tome of Foes includes the derro, I can finally—belatedly—fulfill it.
Derro are small humanoids native to the Underdark. “Equal parts fearful and vicious,” Mordenkainen’s says, “[they] prey on those weaker than themselves, while giving simpering obeisance to any creatures they deem more powerful.” No doubt they want to make the Underdark great again.
With high Dexterity and above-average Constitution but merely average Strength, derro are skirmishers, but not especially mobile ones. Their Intelligence is average, but their Wisdom, for some reason, is in the cellar. This is unusual; the reverse is far more common, especially since Wisdom supports the Perception skill. Not only are they easy to get the drop on, they also have an underdeveloped survival instinct, making them more likely to fight to the death. They are, however, proficient in Stealth, predisposing them toward an ambush strategy.
They have excellent darkvision and Sunlight Sensitivity, so they’ll rarely venture aboveground for any reason, and absolutely never during the day. This, plus their innate paranoia, combine to suggest an intense territoriality—which is to say, not only will they defend their turf viciously, they’ll hardly ever leave it at all, except to try to conquer an adjoining sliver of new territory.
Derro have two weapon attacks, hooked spear and light crossbow. One option with the spear is to knock an enemy prone (presumably by hooking and tripping him or her), which would give an adjacent melee attacker advantage on a follow-up attack. However, a ranged attacker has disadvantage against a prone target, so this doesn’t help the crossbow-wielding derro at all. Even worse: It turns out, if you run the numbers, that even if the first derro in a group successfully hooks and trips an enemy, its allies nearly always do less expected damage, despite having advantage on their to-hit rolls, than the group would do if all of them simply attacked to do damage.
This holds true for any group of two five derro. It takes six or more derro attacking a single opponent in melee for the advantage from hooking and tripping to produce an increase in overall damage, and at that threshold, it only works against unarmored, lightly armored or moderately armored opponents.
Reflecting on this, I think you should consider the hook-and-trip to be an “advanced” derro tactic. Derro have a challenge rating of only 1/4, so you can throw them against even level 1 player characters. Against these PCs, they won’t appear in great enough numbers to do anything but stab. But once your PCs are up around level 5 and higher, they’re going to be fighting hordes of derro, not just patrols and platoons, and in that instance, the first in each group of attackers will hook and trip to try to give the rest advantage. (For the sanity of your players and yourself, use the “Handling Mobs” rule on page 250 of the Dungeon Master’s Guide, and assume that advantage gives +4 to hit.) Assuming they all share the same initiative count, derro wielding crossbows will all shoot first, before any of their enemies fall prone; then the derro with spears will attack.
Alternatively, if you’re more interested in flavor than in optimization, always have the first derro in a group of three or more wielding spears attack to hook and trip. The average difference in damage is less than 1 point, and your players probably won’t do the math on the fly and realize that being flat on their prats doesn’t put them in any more danger than they were in standing up.
After all that, it seems almost anticlimactic to point out that the light crossbow does significantly more damage than the spear—86 percent more, on average. So rather than divide up a derro unit between crossbowmen and spearmen, assume that every derro carries both a crossbow and a spear; that they prefer to use their crossbows over their spears; but that when an enemy rushes them, they switch, and so do their immediate neighbors. Also, if at all possible, they launch their first crossbow volley from hiding, to gain unseen attacker advantage.
Derro paranoia and low Wisdom mean they don’t flee when seriously wounded but rather keep on fighting until they’re down to 1 or 2 hp. At that point, the gravely wounded derro will run, baiting out opportunity attacks—and their erstwhile allies will seize that opportunity to retreat out of melee range themselves and go back to attacking with their higher-damage crossbows. If combat drags on beyond three rounds, all derro will flee the scene, Dashing away. But this is simply a strategic retreat. They‘ll gather some more allies, stalk their opponents and ambush again as soon as they get the chance.
Derro savants are derro with sorcerous ability. Aside from having high Charisma and slightly below-average Strength, they have exactly the same ability contour as a regular derro. Because that below-average Strength makes them even less effective in melee, however, they’ll always attack from range, and other derro will run interference for them in case an enemy tries to close to melee distance.
Lightning bolt is the big gun in the derro savant’s arsenal, but it has the drawback of affecting only a narrow, straight line. Invisibility, however, gives the derro savant the freedom to position itself where it can cast a lightning bolt that nails three or more enemies, if they’re properly lined up. It’s most likely to get this chance if the battle has a well-defined front line. In a more all-over-the-place battle, though, there may never be a good opportunity to cast lightning bolt.
Normally, I’d say, the derro savant should use its 3rd-level spell slots for lightning bolt and nothing else. But I’d also say that because of the length of its area of effect, it’s practically wasted if cast against just one or two enemies. So what about, say, boosting chromatic orb with a 3rd-level spell slot? That would make it do 5d8 damage (22 points, on average) against a single enemy with a ranged spell attack roll, vs. 8d6 damage (28 points, on average) against one or two enemies, with the burden on them to make a Dexterity saving throw, and half damage done even if they succeed. There’s no comparing the two. Chromatic orb falls far short.
Burning hands? At least that one requires a Dex save, does half damage on a success and can affect a second target, but even when boosted to 3rd level, the base damage is only 5d6 (17 points, on average). This one’s a self-defense measure for when the derro savant gets sacked, nothing more. And sleep just doesn’t scale well. So save those 3rd-level spell slots, even if the opportunity to cast lightning bolt doesn’t seem to present itself. The derro savant holds out hope that the moment will eventually come, and when it does, it will be ready.
As for cantrips, the derro savant has two that do damage: acid splash and ray of frost. Ray of frost is better, but neither is that great. The derro savant will use up its lightning bolts and chromatic orbs before resorting to cantrips. Spider climb is useful for escaping in a high-verticality environment, and not much else—and since the derro savant is as unlikely to flee as any other derro, this spell won’t get much use. Ditto sleep, once the PCs are past level 4 or so.
Next: star spawn. No, really, I mean it this time.