Neothelids are products of mind flayer reproduction gone awry. Mind flayers reproduce by hatching thousands of tadpoles and implanting as many as they can in the brains of living hosts. Unimplanted tadpoles must be killed, because if they’re left to their own devices, the tadpoles will grow out of control and dumbly devour every living thing around them, including other mind flayer tadpoles. As they feed and grow, their psionic power grows as well, but the intelligence needed to direct it—which normally comes from the host brain—doesn’t. You can see how this ends: not well.
Gargantuan, clumsily thrashing brutes, neothelids have extraordinary Strength and Constitution but below-average dexterity, subsentient Intelligence but high Wisdom (representing perception and survival instinct, nothing else). It has 120 feet of blindsight, suiting it to any environment but giving it the greatest advantage in subterranean places. It can also detect the presence of intelligent creatures up to a mile away, unless they’re masking their minds with magic.
The combination of high Wisdom and rock-bottom Intelligence indicates a sort of animal cunning, which isn’t the same as flexibility—the neothelid has none of that. Operating purely from instinct, it nevertheless can choose its moment to attack and avoid tangling with creatures of comparable or greater power. It can also detect—imperfectly—which of its prospective victims are weakest and go after them first. And if it’s seriously wounded (reduced to 130 hp or fewer), it will recognize the danger it’s in, break off fighting and Dash away.
Neothelids have Magic Resistance, but more important, they don’t even know what magic is (they don’t know what anything is, only that if it’s moving, they can probably eat it): two good reasons for them not to treat spellcasters any differently from anyone else.
Their two modes of attack are Acid Breath, which recharges, and their tentacles. In almost every instance, I deem recharge abilities to be favored over other attacks, but I find myself second-guessing that with the neothelid. The reason is, Acid Breath doesn’t do anything except damage. Is it self-defense? Is it a way of softening up food before they eat it? The flavor text says, “These creatures can spray tissue-dissolving enzymes from their tentacle ducts, reducing victims to a puddle of slime and leaving only the pulsing brain unharmed.” No. 1, ewww, and No. 2, those are some highly specific enzymes. But No. 3, it does suggest that despite its lack of debilitating side effects, Acid Breath is meant to be part of the neothelid’s digestive process. So, OK, let’s go with that. Since the area of effect is a 60-foot cone, let’s say that the neothelid uses Acid Breath anytime the ability is available and can strike all of the neothelid’s opponents or at least six of them, whichever is fewer (per “Targets in Area of Effect,” Dungeon Master’s Guide, page 249).
The neothelid’s tentacle attack—it gets only one per turn—inflicts both bludgeoning and psychic damage, then requires the target to make a Strength saving throw to avoid being swallowed. It has a two-thirds chance or better of swallowing any character with a +3 Strength save modifier or lower. It can identify which targets are probably easier pickings, but as you make this assessment from the neothelid’s point of view, consider that it can only take into account size and mass, not training. Thus, a fighter with Strength 16, whose Strength modifier is +3 but whose Strength saving throw modifier is +5 or greater, will still be attacked by a neothelid without a second thought, whereas a half-orc cleric with Strength 18 (+4 Strength modifier) would probably be saved for later.
A neothelid can cast levitate at will, although I can’t imagine why it would want to. It also has one daily use each of confusion, feeblemind and telekinesis. Confusion has a range of 90 feet but affects only a sphere with a 10-foot radius, meaning the neothelid will be lucky to catch more than two opponents at once with it. Feeblemind has a range of 150 feet but affects only one target. Telekinesis affects one creature or object within 60 feet.
With each of these spells, I have to ask, what judgment does the neothelid possess that would make it choose to forgo a tentacle or Acid Breath attack—both of which relate closely to its ability to feed—in order to cast one of them? And on what basis would it choose its target(s)? It’s not smart enough to think tactically, let alone strategically.
Let’s come up with a heuristic. Is Acid Breath available, and are all of the neothelid’s opponents (if there are six or fewer) or at least six of them (if there are seven or more) grouped within a 60-foot cone emanating from the neothelid, either before or after its 30-foot move? If so, the neothelid will use its Acid Breath. If not, is at least one opponent within the neothelid’s 15-foot reach, and is that opponent’s Strength modifier (unmodified by saving throw proficiency) +3 or lower? If so, it attacks that opponent with a tentacle and tries to devour it. If not, is there at least one such opponent within 45 feet of the neothelid? If so, it moves, then attacks.
Innate Spellcasting, therefore, is only for those situations in which the neothelid can’t hit enough of its enemies with Acid Breath, either because it’s in the wrong place, they’re in the wrong places, or the ability is recharging; and it also can’t reach an opponent to strike with a tentacle, either before or after moving.
The neothelid has a 15-foot reach and a 30-foot move, so we’re now looking at cases in which an opponent is more than 45 feet from the neothelid at the start of its turn. Adding this distance to the range of each spell, we come up with three circular zones around the neothelid’s starting position, like the rings of an archery target: from 45 to 105 feet away, from 105 to 135 feet away, and from 135 to 195 feet away. Continuing the metaphor, let’s call these the red ring, the blue ring and the black ring. (The golden bull’s eye is where the neothelid can reach an opponent with a tentacle attack.)
In the red ring, telekinesis is consistent with the neothelid’s rudimentary “Ge’ in mah mouf” mentality. It moves as far as it can—30 feet—toward an opponent in the red ring, then casts telekinesis to try to pull that opponent 30 feet toward itself. The result of the pull is determined not by a Strength save but by a contest between the neothelid’s Wisdom modifier (+3) and the target’s Strength modifier. Therefore, the neothelid won’t try this against a target with a positive Strength modifier—it’s got to be zero or negative. If there is such a target in the red ring, then once the neothelid takes telekinetic hold of him or her, it will maintain that hold until it comes close enough to make a tentacle attack, then lands one successfully.
Confusion requires at least two targets within 20 feet of each other, each in either the red ring or the blue ring. A neothelid can make reasonably accurate guesses about a creature’s Strength, but it has no way to assess a creature’s Wisdom, so it’s not going to be picky about who it targets with this spell, as long as they’re properly situated. Of course, if it can get more than two targets in the radius of effect, it will. After casting the spell, it maintains it until it’s landed a tentacle attack on each affected target.
Feeblemind is the neothelid’s last resort, cast only if none of the aforementioned criteria is met. It’s an attempt to hobble a single target, who can be anywhere within the black ring or closer. However, there’s not really any good target selection basis that the neothelid would be aware of, since it can’t tell one humanoid from another, let alone spellcasters from non-spellcasters, or other spellcasters from wizards. There’s also the consideration that feeblemind’s range exceeds the range of the neothelid’s blindsight. (In fact, its blindsight reaches only into the middle of the blue ring, at least from the start of its movement.) Maybe it lashes out at someone who’s done damage to it from a distance. Maybe it reacts to an attempt to flee. Or maybe it just targets whoever’s closest. Let’s just hope that the circumstances are right for one of the neothelid’s other attacks, and we never get this far up the decision tree.
If a neothelid is sustaining telekinesis or confusion, it drops the spell immediately when it takes enough damage to compel it to flee.