Maurezhi, according to Mordenkainen’s Tome of Foes, are demons formed from the corrupted souls of elves to lead packs of ghouls and ghasts. The connection to elves is interesting, because as I note in my article on ghoul and ghast tactics, whether or not their claw attacks have an effect on elves is a key feature distinguishing ghouls from ghasts. (Ghouls’ claw attacks have no paralyzing effect on elves; ghasts’ claw attacks do.)
Upon consuming the corpse of a humanoid it’s slain, a maurezhi has a brief window of opportunity during which it assumes his or her appearance and can convincingly pass as that person, with the help of its proficiency in the Deception skill. Almost immediately, however, this body begins to rot away, and after just a day, something is clearly not right; by a few days later, the maurezhi sheds it, like a skin it’s outgrown. The ideal maurezhi encounter therefore takes place very soon after it’s assumed a new appearance, and any delay is going to have an effect on its strategy.
Maurezhi have high Charisma, but this doesn’t figure into any of their attacks, only their deception skill. Combat-wise, they’re shock attackers, with exceptional Dexterity that functions as their primary ability for both defense and offense. Since ghouls and ghasts are also shock attackers, and since maurezhi will typically be encountered leading packs of them, there should be enough baddies to go around for an entire group of protagonists; the ghouls and ghasts go straight for the ones they want to eat, while the maurezhi zeroes in on any other opponent who might present an obstacle to that and tries to take him or her out in a round or two.
Because of their Magic Resistance; their resistance to cold, fire, lightning and necrotic damage; and their immunity to being charmed, maurezhi are good at taking out spellcasters and will often go after these enemies first. Immunity to poison damage and the poisoned condition also makes them good for neutralizing rogues. They have 120 feet of darkvision, which is common among subterranean creatures. They absolutely prefer to initiate combat underground or at night, not in sunlight.
Raise Ghoul is a recharge ability, which suggests that the maurezhi will use it whenever it can—provided that there’s a dead ghoul or ghast to use it on. Aside from this action, the maurezhi has a Multiattack action that includes one bite attack and one claw attack. The claw attack works similarly to a ghast’s, with a paralyzing effect (and no exception for elves). Because attacks against a paralyzed creature have advantage, the maurezhi will claw first, then bite. The bite attack “can drain a victim’s sense of self,” sucking away points of the target’s Charisma, and reducing a Charisma score to 0 results in instant death and eventual ghoulification. This offers no tactical advantage, but its distinctiveness suggests that it’s something the maurezhi wants very much to do.
Since everything about the maurezhi seems to revolve around procuring victims for ghouls and ghasts and making new ghouls out of the victims who don’t get eaten, I think we can conclude that the main thing a maurezhi does with its false appearance is try to lure unsuspecting dupes into ghoul/ghast dens and trap them there. Despite being a shock attacker, the maurezhi isn’t unusually fast, so these dens should have enough room to maneuver around freely without being so large that it can’t reach its desired targets in a single move—probably around 40 to 60 feet in each linear dimension.
While the ghouls and ghasts paralyze and feast on whomever they can, the maurezhi goes around playing a cleanup role, using its own claw/bite Multiattack (in that order) to disable any opponent whom the ghouls and ghasts aren’t having any luck with. If all opponents are occupied, it will help a ghast try to take down a strong target. When a ghoul or ghast goes down, the maurezhi takes a break from its own activities to raise it again. It doesn’t worry a great deal about opportunity attacks: its armor class is middle-of-the-road, but its resistance to physical damage from nonmagical weapons is enough to make the difference.
Being demons, maurezhi don’t need to worry if their physical vessels are destroyed: their souls will bamf back to the Abyss, and their bodies will re-form there. However, they’re not going to keep fighting if the ghouls and ghasts they’re with have had enough. When the last ghoul or ghast decides it’s done fighting, either because it’s wounded or because it has a paralyzed victim and is hauling him or her away, the maurezhi covers their retreat, continuing to Multiattack against any opponent who attempts to pursue, Dodging instead if no one does.
The dybbuk, another undead-adjacent demon, originates in Jewish folklore as a malicious spirit (דיבוק dibbūq) that “adheres” to living people and possesses them. In some tellings, the spirit is a demon; in others, the ghost of a deceased person. Mordenkainen’s classifies the dybbuk as a demon, but its modus operandi is similar to the dybbuk of folklore, a major difference being that it can possess only corpses, not living beings. (Another major difference is that an unadhered dybbuk appears as a “translucent flying jellyfish.”)
Dybbuks have extraordinary Dexterity, very high Constitution and a 40-foot flying speed with hovering capability; this, plus a raft of damage resistances, makes them mobile skirmishers that fly from target to target without worrying unduly about opportunity attacks. They also have very high Intelligence, but their spellcasting ability is Charisma, which is the lowest of their mental abilities, so they’ll generally favor their Tendril attack over spellcasting if they’re really trying to hurt someone.
Usually, a group of player characters will encounter a dybbuk when it’s already possessing a corpse and acting out through it in impertinent ways. As I read its stat block, it has no access to its Innate Spellcasting while possessing a corpse, nor does it have its Magic Resistance or any of its damage resistances, so if it’s attacked, its first recourse is to try to scare its attackers off using its Violate Corpse feature. But this is another of those features, like a dragon’s Frightful Presence, that either works the first time or never works at all, and that no longer works once a target shakes it off; it also has a low saving throw DC. So there’s a good chance that, against a group of PCs, it won’t have the effect the dybbuk desires.
Its second recourse, then, is to run. It doesn’t care all that much about the body it’s possessing, so even though it has the Intelligence to Disengage, it won’t bother; it just Dashes.
Alternatively, if you’ve constructed this as a Deadly encounter, it may decide it can take the party head-on, in which case rather than try to run away, it ditches its counterproductive corpse-body (bonus action), regains access to all its resistances and immunities—and its Innate Spellcasting—and lashes out with a necrotic tendril against the nearest unarmored or lightly armored opponent. It then flies 20 feet, ideally upward out of reach. Knowing how low its spell save DC is, it doesn’t try to cast fear or phantasmal force against a group of PCs unless it notices after a round or two that none of them has tried to use any kind of magic against it. (Mostly, it reserves these spells for commoner NPCs.) A dybbuk forced out of its body, either because the body is destroyed or because it was expelled using magic such as the Break Enchantment function of dispel good and evil, follows the same tactic, relying on tendril attacks and staying out of melee opponents’ reach between turns.
Dybbuks are loath to give up their mischief—or their corpses. If a dybbuk is moderately wounded (reduced to 25 hp or fewer), it flies down to the corpse it previously possessed, snatches it up and casts dimension door, taking the corpse with it! Once it’s safely away, it uses Possess Corpse again at the earliest opportunity. (If the corpse was destroyed, the dybbuk doesn’t bother to take it along—it can find another.)
Dybbuks can be—and should be—a real pain in the neck. As soon as any encounter turns significantly against them, they dimension door away and either re-inhabit the same corpse or find a new one, therein to commit further indecencies. As long as the PCs are looking for them, they have the sense to lie low and—if they’re in a new host body—act normal, using their expertise in Deception to maintain their disguise until their hunters have gone away. Then they’re right back to their old tricks.