Gonna do my best here with the drow arachnomancer, but please forgive me if I screw up, like, half a dozen different things. I’m operating with two levels of exhaustion, and I’m not even the one doing most of the work. My wife is a boss.
Arachnomancers are drow warlocks that can shapeshift into or out of a Large giant spider form as a bonus action and can continue to speak and cast spells in their spider form. Because they’re warlocks, unlike most monsters with spellcasting ability, they cast all their spells as if using a 5th-level spell slot, but they’re also limited to three leveled spells per encounter (not counting darkness, dominate monster, etherealness, eyebite, faerie fire and levitate, each of which they can cast once per day without spending a slot, and dancing lights, which they can cast at will). Concentration, of course, is going to govern which of these spells they can cast, so we’re going to look for sustained spells that synergize with multiple instantaneous spells.
Also, since these are warlocks we’re talking about, we want to find out what works well with eldritch blast. Although it isn’t stated explicitly in the stat block, because the drow arachnomancer is a 16th-level spellcaster, eldritch blast fires three bolts per casting, for a total of 3d10 force damage. In terms of damage dealt, this can’t compete with either its humanoid-form Poisonous Touch attack or its spider-form Bite attack. However, based on its ability contour—extraordinary Intelligence, very high Dexterity and Charisma, merely above-average Constitution, average Strength—we can infer that the arachnomancer is a long-distance spellslinger that would much prefer to stay out of melee if it can. Thus, Poisonous Touch and Bite are primarily self-defense measures, secondarily shock attacks.
The arachnomancer has proficiency in Constitution, Intelligence and Charisma saving throws, but not Dexterity or Wisdom. It has advantage on saving throws against being charmed, but not against other forms of will-focused magical manipulation, such as hold person or confusion. Thus, it has good reason to feel nervous around other spellcasters and will try to kill or shut them down first.
It also has proficiency in Perception and Stealth, the classic drow ambush combo. Its Wisdom is high enough that it’s going to intuit when a band of trespassers is too strong for it and its squad to wreck quickly in such an attack, but its Intelligence is high enough that it knows exactly which glass cannon to smash in order to swing the odds in its own favor. Any ambush led by an arachnomancer should feel like a worst-case scenario to its enemies. An arachnomancer will parley, but only to the extent that it will accept its enemies’ surrender rather than insist on systematically slaughtering them.
The drow arachnomancer, like all drow, has 120 feet of darkvision and Sunlight Sensitivity and thus operates strictly at night and underground. It also has blindsight, so darkness is highly advantageous to it. On the other hand, it’s well aware that most of its likely companions lack blindsight and will be just as hobbled by darkness as its enemies are, making it a highly situational spell for the arachnomancer to use.
Intriguingly, the arachnomancer possesses its climbing speed and Spider Climb and Web Walker features regardless of whether it’s in humanoid or giant spider form. This gives it some interesting positioning options as well as an incentive to spam an entire battle zone with webs, created by itself, its companions or both. It also means that which shape it assumes is largely a function of which of its attack actions is more desirable, given the circumstances. The one unalloyed advantage of its giant spider form is that it can spin webs as a targeted action that doesn’t require concentration, rather than as an area-effect spell that does—good for setting up combos with allies.
Examining which spells require concentration and which ones don’t, we come to the unpleasant discovery that the vast majority of the arachnomancer’s spells do require it; the only ones that don’t are etherealness, dimension door, dispel magic, chill touch, eldritch blast and poison spray. Etherealness and dimension door are transportation spells, used at the beginning or end of a fight, not in the middle. Dispel magic is crisis response. Poison spray is an all-or-nothing cantrip targeting Constitution, offering little or no opportunity for a tactical combination. Therefore, any synergy we find is going to have to involve chill touch, eldritch blast, Poisonous Touch, Bite and/or the attacks of the arachnomancer’s allies.
- Dominate monster targets Wisdom; is weak against elves, gnomes, berserker barbarians, and Devotion paladins and their companions; and only ever affects one target. This is a high-risk spell with not a lot of upside, except as a narrative excuse to have a non-drow, non-spider monster fighting on the arachnomancer’s side. And that usage offers no synergy.
- Eyebite also targets Wisdom, but it doesn’t charm its target. It allows the arachnomancer some synergy of its own or to take down multiple enemies, one by one, but not both. It seems most useful either for guarding a location (frightened effect) or for dropping foes unconscious for subsequent capture (asleep effect), and remember, the latter effect doesn’t work on elves.
- Conjure animals summons only spiders with a challenge rating between 1/4 and 2, which means either 16 giant wolf spiders or two regular giant spiders (remember, it’s being cast at 5th level). If the arachnomancer has an ally that can blanket the area with webs, go with the giant wolf spiders to get loads of attacks with advantage. If not, two lousy giant spiders aren’t worth it.
- Crown of madness doesn’t scale with level and has the same drawbacks as dominate monster, and unless the arachnomancer’s enemies are in a confined space, it’s too easy for them to get out of the target’s reach. Pass.
- Fear isn’t so good for hurting enemies, but it’s excellent for driving them away—or, if they’re trapped in a confined space, for making it hard for them to fight back. Still, this one doesn’t scale, either.
- Fly can be used on up to three creatures, but it’s mainly useful as a speed doubler for drow elite warriors and shadowblades, since the arachnomancer already has nifty movement abilities of its own.
- Giant insect has no meaningful advantage over conjure animals. You can get three giant spiders with it rather than two. Whooooooo.
- Hold monster cast at 5th level might as well be hold person cast at 2nd level. Don’t get me wrong: Hold [entity] is the best spell since Pepperidge’s apportioned loaf, synergizing with eldritch blast, Poisonous Touch, Bite and pretty much every other attack the arachnomancer’s allies might have. But as hold monster, it paralyzes only one target, whereas as hold person, it could have paralyzed four. Feh.
- Insect plague is simply an area-effect meat grinder that imposes no debilitating condition. Good for stacking damage on damage, but it can’t be used to set up a tactical combination. It can, however, be used as the second part of a tactical combo if the first part grapples or restrains its targets, so that they can’t get out of the area of effect. An ally with a web spell could fill this part of the bill.
- Invisibility is good for a first attack that’s guaranteed to come from hiding, but it does nothing beyond that. (ETA: Not exactly. It’s also good for slippery disengagement once the arachnomancer has finished off an enemy it’s asassinating with a melee attack, since once this work is done, it would prefer to retreat to longer range again.)
- Vampiric touch is a nasty substitute for Poisoned Touch: it does 5d6 necrotic damage rather than 8d6 poison damage, but on the flip side, it also heals the arachnomancer. In the best-case scenario, the arachnomancer uses it against a paralyzed opponent, but the only likely source of that condition is its own hold monster, which it can’t cast at the same time. (Well, there is one other possible case: if a spider ally has reduced an enemy to 0 hp with its poisoned bite, thereby also paralyzing him or her. In this scenario, vampiric touch seems like massive overkill—literally.)
- Web is more useful to the drow arachnomancer than it often is for other spellcasters, thanks to its movement abilities. Surprisingly, this unboostable 2nd-level spell may be one of the arachnomancer’s best sources of tactical synergy, since it imposes the restrained condition on targets who fail their saving throws. This gives the arachnomancer and its allies advantage on all their attacks, including chill touch, eldritch blast, Poisonous Touch and Bite.
- Witch bolt could be described as “concentration optional” if not for one thing: casting it nevertheless requires the arachnomancer to drop any other sustained spell it’s concentrating on. Bogus.
- Darkness, as mentioned elsewhere, is fine for the drow arachnomancer but problematic for most of its allies, so it will use this spell only if it can cover an area that includes itself, any enemy or enemies it wants to attack, none of its allies, and no enemy that it needs its allies to attack. This makes it highly situational.
- Faerie fire is always super-useful; it’s also an innate ability of every drow. One of the arachnomancer’s allies can cast this instead.
- Levitate and dancing lights are wastes of the arachnomancer’s limited concentration.
Concentration not required
- Etherealness and dimension door are primarily escape hatches, since the arachnomancer can’t bring its whole squad along. If it’s accompanied by a drow VIP, it will favor dimension door over etherealness and bring the VIP along.
- Dispel magic can be used to auto-negate a spell of 5th level or lower, which is handy, although it does mean the arachnomancer can’t use its action to deal damage instead. For this reason, it casts dispel magic only to eliminate an effect that’s causing it very serious trouble, with respect to either its ability to deal damage or the ability of it and its allies to survive.
- Chill touch, at the arachnomancer’s level, does 3d8 necrotic damage. This is only 3 less, on average, than eldritch blast, and it comes with the benefit of suppressing healing. If the arachnomancer is out to straight-up assassinate a single spellcaster, it uses this damaging cantrip instead of eldritch blast.
- Eldritch blast, in contrast, is better against targets of opportunity, or against a force that lacks healing capability. With its Intelligence 19, the drow arachnomancer can size up its foes and know whether or not this capability exists.
- Poison spray compares favorably with chill touch and eldritch blast only against low-Constitution, non-dwarf, non-high-level-monk targets. With DC 16, the saving throw is just a little too easy for higher-Constitution enemies to succeed on. Also, because it’s resisted with a Con save rather than a Dex save, there’s no way to work it into a tactical combo.
So we have a few different possible game plans for the drow arachnomancer. If it needs to take out a single powerful but fragile spellcaster fast, it can cast hold monster as its first action, then rush in and deliver a vicious shock attack with Poisonous Touch or Bite while its allies keep other enemies tied up. If there are multiple spellcasters that need to be confronted at once, it can web them, granting itself and its allies advantage on all weapon and spell attacks. If it’s accompanied by other drow mages, it can let them web up the area, then cast insect swarm on top of its restrained enemies, then attack targets of opportunity from afar with chill touch or eldritch blast (the three energy beams can be aimed all at the same enemy or at different ones). Or, after its allies blanket the combat zone with webs, it can cast conjure animals and summon a pack of speedy and fierce giant wolf spiders, which attack as a mob and all gain advantage against restrained targets, and only then start lobbing its cantrips. Finally, fighting primarily non-spellcasters in a confined space, it can cast fear as its first action to hobble their counterattacks while its allies engage them in melee, then take potshots with chill touch or eldritch blast.
Drow arachnomancers, like other drow, are zealots, but they’re highly intelligent zealots who keep their objective front and center in their minds. Damage won’t cause them to flee, but if their objective can no longer be achieved, retreat is a reasonable alternative, especially if it allows them to warn of coming dangers and/or summon reinforcements. Being highly hierarchical and having the kind of contempt for their underlings you’d expect from chaotic evil creatures, an arachnomancer that decides it’s time to leave won’t hesitate to do so on its own, leaving its erstwhile allies behind.
Next: drow shadowblades.