Aside from devils and demons (and their lawful and chaotic fiend-kin), there remain four neutral evil fiends that receive significant updates in Monsters of the Multiverse: three types of yugoloths (hydroloths, yagnoloths and oinoloths) and barghests.
Hydroloths, which in traditional cosmology dwell in the waters of the River Styx and in the Multiverse could show up in any body of water you like, I suppose, gain substantial buffs to their melee damage—the slashing damage of their Claws and the piercing damage of their Bite are both changed to force damage, and each does another 2d10 psychic damage in addition—and lose phantasmal killer and water walk from their list of spells. Phantasmal killer was a trap option to begin with, so this loss is really a gain, because the temptation to use it is taken away. Water walk served little purpose for the hydroloth itself but could have been sold as a service to other creatures so that they wouldn’t suffer memory erasure by falling into the Styx, for instance. Presumably for the sake of reducing Dungeon Master decision paralysis, that spell has been removed, but knowing what you know, you might choose to keep it for a specific plot purpose. The hydroloth’s tactics, however, are unaltered by these changes.
The one disappointing thing about the revisions to the yagnoloth is that the flavor text no longer refers to its “brutally powerful giant appendage” (it’s been changed to the boringly specific “brutally powerful giant arm”). Aside from that, and the fact that the aforementioned brutally powerful appendage now deals force rather than bludgeoning damage, the yagnoloth has a more flexible Multiattack: In addition to Massive Arm/Electrified Touch and Massive Arm/Teleport (or Teleport/Massive Arm), there’s also a Massive Arm/Battlefield Cunning option.
When Battlefield Cunning precluded Multiattack, it was necessary for the yagnoloth’s entourage to deal a total of at least 50 damage. Now it only has to deal 27 damage to make it competitive with Electrified Touch, which means that in addition to canoloths, nycaloths are good enough sidekicks as well, especially if they use their Greataxe attacks. So are dhergoloths, as long as their Flailing Claws attack isn’t on cooldown. In fact, any combination of these particular yugoloths makes Massive Arm/Battlefield Cunning preferable to the yagnoloth’s other Multiattack options, which makes the attack’s recharge a lot easier to justify. The old conditions on the use of Battlefield Cunning no longer apply; this particular Multiattack is now better than any of the other options the yagnoloth has. Aside from this difference, the heuristic described on pages 397–98 of MOAR! Monsters Know What They’re Doing stands, with Multiattack (Massive Arm/Battlefield Cunning) falling between Life Leech and Multiattack (Massive Arm/Electrified Touch).
As constructed in Mordenkainen’s Tome of Foes, the oinoloth had a problem: It took necrotic damage from the areas of blight created by its own Bringer of Plagues bonus action, as did other yugoloths. Monsters of the Multiverse fixes that problem. Bringer of Plagues now deals poison damage, to which all yugoloths are immune. Thus, both the oinoloth and any other yugoloths that accompany it have complete freedom to move around the battlefield, even when the oinoloth is spreading plague all over the place. Not only that, the obvious ploy of creating a plague zone centered on itself, which used to be rendered impractical by the necrotic damage it dealt to the oionoloth, is now not only feasible but just as smart a move as you’d think. I think this ranks as one of the most important bug fixes in all of Monsters of the Multiverse.
Before I look into the other implications of this change, let me quickly enumerate the others: The oinoloth’s hit dice used to be d10s, which may have been an error, since it’s only a Medium creature. They’re d8s now, consistent with its size; and it has two more of them, landing its new average hit point maximum (119 hp) pretty close to its old one (126 hp). If you wanted to make it even closer, you could give it 15 hit dice rather than 14, for an average of 127 hp. The Transfixing Gaze action is history, as are wall of fire and wall of ice; in lieu of these mobility deniers, the oinoloth can cast hold monster at will. The upshot of this change is that it can choose to keep a single enemy rooted in place for as long as it keeps failing saving throws, whereas before it had to target that enemy with Transfixing Gaze turn after turn. The action was also useless against targets that couldn’t be charmed. Finally, the targeted creature is now not merely restrained but paralyzed. Casting hold monster on one turn, then closing and Clawing on the next is not only a feasible combination but a potent one against an enemy that refuses to stand still.
Whereas the oinoloth’s Multiattack used to comprise Transfixing Gaze and two Claw attacks, it now comprises two Claw attacks and either Spellcasting (including hold monster, but also feeblemind, globe of invulnerability, darkness, dispel magic or invisibility) or Teleport. That’s huge: none of those spells requires the oinoloth to forgo its Multiattack anymore.
These changes transform the oinoloth’s tactics completely. Without the need to steer clear of its own blight zones, the oinoloth is free to express its nature as a more static combatant, daring its foes to come to it. Its fundamental approach is to pin down an enemy within 45 feet of it with hold monster, close with it, aim two Claw attacks at its paralyzed foe (auto-critting on each hit), then exude a blight zone centered on itself (bonus action), daring its victim’s allies to do something about it. Each time Bringer of Plagues recharges, it can repeat this sequence.
Against foes whose Wisdom saving throws are too good for hold monster to work (something that the intelligent but not extraordinarily intelligent oinoloth can determine only after a couple of rounds’ observation), there’s always feeblemind or a simple Teleport/Claw/Claw Multiattack. With an attack sequence as strong as the oinoloth’s new default, it no longer makes routine sense to open with globe of invulnerability, even though it doesn’t consume the oinoloth’s entire action to cast. Instead, the oinoloth casts that spell when it already has a paralyzed victim to pick on and is getting plinked with spells by its victims’ friends, or if it knows to begin with that it’s going up against several spellcasters.
Even though the oinoloth still has darkness, because it nullifies all sources of attack roll advantage, it’s not an ideal spell to cast unless hold monster straight-up isn’t working at all against the oinoloth’s opponents. In fact, the only time—and the perfect time—for an oinoloth to cast darkness is when all its attempts to paralyze its opponents are failing. Invisibility is for retreating when the oinoloth is seriously wounded on its home plane, and dispel magic for highly inconvenient spells of 3rd level or lower. Otherwise, it’s hold monster all day. The oinoloth’s target selection and retreat criteria remain the same.
Did barghest encounters drag on too long? Perhaps. Its defenses have been nerfed quite a bit: one-third of its hit dice taken away, no more fire resistance. At the same time, its offensive power has been cranked up. It now has a Multiattack that lets it make both a Claw attack and a Bite attack in a single turn, and it gains a disturbing new trait, Soul Feeding, that threatens to make the death of one of its opponents permanent if it’s left with enough time to dine. That certainly raises the stakes. However, none of these changes compels it to change its decision-making in any way. Its tactics remain the same.
Next: celestials, fey, elementals, constructs, oozes and beasts.