It took me a couple of tries to get through the flavor text on the nightwalker in Mordenkainen’s Tome of Foes, but here’s what it seems to boil down to: If some schmuck is dumb enough to try to visit the Negative Plane, which has even less to recommend it as a destination than Philadelphia International Airport, the tradeoff is that a nightwalker is released into the material plane, and the visitor can’t leave the Negative Plane until the nightwalker is somehow persuaded to go back. How can it be persuaded to go back? “By offerings of life for it to devour.” How many such offerings are necessary? It doesn’t say. What do nightwalkers want? “To make life extinct.” So the idea here is to convince a nightwalker to abandon the place where it has plenty of life energy to devour by giving it life energy to devour? Try throwing bagels to raccoons and see how quickly they go away.
As if this arrangement weren’t bad enough for our traveler, destroying the nightwalker traps the traveler on the Negative Plane forever. In short, in an entire universe of bad ideas, going to the Negative Plane for any reason is quite possibly the worst. If you’re creating a nightwalker encounter, though, someone went through with this execrable half-baked plan, and now your player characters are the ones who have to deal with the consequences.
With extraordinary Strength and Constitution, nightwalkers are brutes, but they’re some of the nimblest brutes in the Dungeons and Dragons menagerie: their Dexterity is also extraordinary, though not quite as high as their Strength and Con. Their mental abilities, in contrast, are weak, with below-average Wisdom the highest of the three. They’re indiscriminate in their target selection and operate on instinct, without any flexibility in their tactics.
They have a huge repository of hit points, and they’re either immune or resistant to nearly every type of damage; the only exceptions are force and radiant damage, along with physical damage from magic weapons. They can’t be exhausted, frightened, grappled, paralyzed, petrified, poisoned, knocked prone or restrained—but can be blinded or stunned.
Nightwalkers have 120 feet of darkvision. Their name is meant to be taken literally. They don’t go out during the day. There’s no percentage in it for them.
Well, the first part of their name is meant to be taken literally. The second part, less so: Nightwalkers have a base movement speed of 40 feet but also a flying speed of 40 feet. This, combined with the illustration in Mordenkainen’s, makes me imagine a creature that glides rather than walks—or that “walks” even when its feet aren’t touching the ground. In fact, it might be interesting to ratchet up the weirdness factor by having it glide without moving its feet when it’s “walking” and make walking motions when it’s flying.
Annihilating Aura and Life Eater are passive features. The one thing we can infer from Annihilating Aura is that, because it has a 30-foot radius, the nightwalker tries to position itself within 30 feet of all its potential victims (or at least six of them, if there are more than that) at the end of its turn. Now I’m imagine a constant gliding, wheeling movement pattern, in which the nightwalker never stops moving entirely but rather strikes its foes en passant on its way to its final desired position.
The nightwalker’s Multiattack comprises either two uses of Enervating Focus, a melee attack, or one of Enervating Focus and one of Finger of Doom, a ranged attack with a long recharge. I said above that the nightwalker is indiscriminate in its target selection, but that’s not entirely true. It doesn’t distinguish between one life and another, but it does respond to position and condition. Finger of Doom has a 300-foot range, but the paralysis it incurs offers sweet synergy to the nightwalker, which has only until the end of its next turn to exploit it. Therefore, when it uses Finger of Doom, it targets a creature no farther away than 40 feet plus however many more feet of movement it has remaining on its current turn. If there’s an opponent within this radius who’s affected by Annihilating Aura as well, or against whom the nightwalker has advantage for some other reason, such as his or her being unable to see in the dark, that opponent is preferred.
The nightwalker uses Finger of Doom as part of its Multiattack whenever it has this ability available and has advantage against any opponent within this 40-foot-plus-remaining-move radius. If it can reach the target of Finger of Doom in the same turn, it uses Finger of Doom first. If it succeeds in paralyzing its target, it closes the distance and uses Enervating Focus against that target. If it doesn’t succeed in paralyzing its target, it instead closes on another target nearby, preferably one it can attack with advantage, and attacks that target with Enervating Focus.
What if it can’t reach its Finger of Doom target in the same turn? Then it starts moving in that target’s general direction and uses Enervating Focus on some other target it doesn’t have to go too far out of its way for—again, preferring a target it can attack with advantage.
If the nightwalker doesn’t have Finger of Doom available (either because it hasn’t recharged or because it doesn’t have advantage on any attack with that feature), then it attacks with Enervating Focus twice. If there’s only one opponent within 40 feet whom it can attack with advantage, it makes both attacks against this opponent. If there’s more than one, it may attack one twice, or it may attack two once each, but it always makes these attacks along a path toward the point where it can reach as many opponents as possible with its Annihilating Aura at the end of its movement.
Nightwalkers don’t worry about opportunity attacks except from opponents with magic weapons. They don’t knowingly and willingly move within reach of these weapons—and usually don’t have to, since their own melee attack has a 15-foot reach. If an opponent wielding one of these weapons charges a nightwalker and engages it in melee, it rises up into the air, out of that opponent’s reach, and then focuses all its attacks on that opponent. If attacked by multiple opponents with magic weapons, it stays out of their melee reach; divides its attacks among them, using Finger of Doom against ranged attackers wielding magic weapons or ammunition first and foremost; and disregarding opponents who aren’t wielding magic weapons or casting spells that inflict force or radiant damage.
A nightwalker doesn’t flee, regardless of how much damage it takes. It has no evolved origin and no survival instinct, and what flicker of sentience it possesses gets a kick out of knowing that its destruction will cause some poor idiot to be trapped on the Negative Plane forever.