We now return you to your regularly scheduled monsters. Today, the upper management of the demonic hierarchy: the type 4 nalfeshnee, the type 5 marilith, and the type 6 balor and goristro.
As mentioned before, demons can’t be killed on the prime material plane—or on any other except their home plane, the Abyss. Any demon killed elsewhere simply re-forms there. Therefore, demons fought on any other plane don’t fear death and won’t retreat or flee even when seriously injured. They inflict as much injury and damage as they can until they’re destroyed.
Also, all demons are (at a minimum) resistant to cold, fire and lightning damage and immune to poison, and at this level, they’re all immune to physical damage from normal weapons as well. Additionally, they have either darkvision or truesight, giving them advantage at night and underground.
Nalfeshnees are often encountered in command of lesser demons, rather than on their own. They follow a straightforward brute profile, with extraordinarily high Strength and Constitution—but also extraordinarily high Intelligence, although their Wisdom is in the high-average range. The latter indicates a monster that can plan and coordinate, adapt on the fly, accurately assess its enemies’ strengths and weaknesses, and choose targets for itself and its minions accordingly. It may even refrain from combat in favor of parley if it knows it’s outmatched, but with demons, the embodiment of chaotic evil, that’s a big maybe—because, as mentioned, their drive is to destroy, and death is only an inconvenience to them. It would probably take a chaotic evil character to negotiate any useful cooperation out of a nalfeshnee. Or any kind of demon, really.
The nalfeshnee’s most distinctive feature is its Horror Nimbus, an ability that requires a Recharge and is usable, on average, one turn out of every three. Like a dragon’s Frightful Presence, this is one of those features that a creature can hit each of its enemies with only once, because after that, if they’re not affected on the first try, they’re not going to be affected at all in the same encounter. Also, uniquely, the Horror Nimbus is part of the nalfeshnee’s Multiattack. Consequently, it’s most useful against whichever opponent the nalfeshnee is engaged in melee with at the moment. So although the nalfeshnee could easily expect to hit three opponents at once with it (based on “Targets in Area of Effect,” Dungeon Master’s Guide, p. 249), not only will it not try to position itself to affect as many opponents as possible, it will do just the opposite: try to position itself to affect only its current opponent, or the opponent that it’s about to engage.
Because of its flying ability, the nalfeshnee will generally fight from the air, flying down toward an opponent, using its Multiattack action, then flying back up. With an armor class of 18 and 184 hp, it has very little to fear from opportunity attacks. That being said, however, if an enemy somehow manages to land two opportunity attacks in a row, the nalfeshnee will wise up and start fighting that particular enemy toe-to-toe on the ground.
Between its Magic Resistance and Teleport features, the nalfeshnee is good at taking out spellcasters who are giving its minions trouble. (If they try to flee when the nalfeshnee appears, it will use its bite attack for its opportunity strike.) Aside from that, it will focus on the opponents who pose the greatest threats to itself and its minions. In most cases, any player character using a magic weapon (especially one that does extra damage against demons or evil creatures in general); inflicting acid, thunder, radiant, necrotic or psychic damage; or casting spells that require Dexterity saving throws to avoid will fall into this category.
Mariliths are demonic Cuisinarts, wielding six longswords and possessing a prehensile tail to boot. This tail can grab and restrain an enemy, giving a marilith advantage on every attack against him or her. In addition, mariliths’ Reactive and Parry features together give it an effective AC 23 against melee attacks. Mariliths love melee combat. That being said, their physical abilities follow a skirmisher profile, and they have a 40-foot movement speed, so they’re not necessarily going to sit in just one place and fight whoever comes to them.
Mariliths have exceptionally high Intelligence and very high Wisdom, so they’re also good at planning, coordinating, adapting and selecting targets. Like nalfeshnees, they’re often commanders of other demonic troops, and they assess threats according to the same criteria that nalfeshnees use. They also share nalfeshnees’ Teleport feature, and with their fell Multiattack, they can make enemy spellcasters’ lives very, very short. If a spellcaster tries to run away, a marilith will lash out against him or her with its tail and put a quick stop to that.
In melee combat, their first attack will always be a tail attack; if this attack hits, grappling the target, all longsword attacks for the remainder of the Multiattack will be against the restrained enemy. On subsequent turns, if an enemy is grappled by a marilith’s tail already, it will divide its attacks equally among that enemy and any other(s) engaging it in melee, aiming any remaining attack(s) at the grappled enemy.
Balors (don’t ever call ’em “balrogs”—they hate that, and so do the Tolkien estate’s lawyers) fight as generals at the head of demon armies. With their brute ability profile and their Fire Aura feature, which burns every creature in an adjacent square or hex, they not only love toe-to-toe combat, they’re happy to be surrounded by as many enemies as possible. And like nalfeshnees and mariliths, it’s very good at assessing its enemies and targeting the biggest threats.
The balor’s Multiattack consists of a longsword strike and a whip strike; it can use the whip strike to snag an enemy as far as 30 feet away and yank him or her into its hot zone, then follow up with its longsword. On subsequent turns, it may continue to use this whip-sword combo against the same opponent; finish off a seriously injured opponent with its sword, then use its whip to seize another; or continue to fight an opponent already within reach with its sword while using its whip to pull more enemies into its Fire Aura. Or, like the nalfeshnee and the marilith, it may Teleport to the position of an enemy spellcaster and make an opportunity attack with its longsword if that enemy tries to get away. Although they can fly, balors generally fight on the ground, because their Fire Aura takes effect at the start of their turns, so they need to be adjacent to their enemies at that moment.
The Monster Manual isn’t clear on whether a balor’s Death Throes applies to the destruction of its physical form on a plane other than the Abyss, or only to its final destruction on its home plane. I’m going to assume this feature applies to its physical form on any plane [now officially confirmed], in which case a seriously injured balor (reduced to 104 hp or fewer) will try to place itself within 30 feet of as many enemy creatures as it can, in order to take them with it when it goes.
Finally, there’s the goristro, described by the MM flavor text as a “living siege engine.” Like the other high-level demons above, its physical ability profile is that of a brute. Unlike them, it’s stupid, operating purely and solely from instinct. It can recognize a threat the same way an animal can and choose targets accordingly, but it can’t make detailed assessments about PCs’ abilities or features, nor does it adapt to changing situations.
The goristro’s most distinctive ability is its Charge, which requires it to move at least 15 feet straight toward a target, then gore it, potentially knocking it prone. A failed Strength saving throw also means the target is thrown 20 feet away, meaning the goristro can charge it and knock it down again the following turn! (This is a lot of fun for goristros. They call it “head golf.”) The goristro’s Charge does massive damage—14d10 + 7 hp piercing on a hit, or 84 hp on average. In contrast, its fist/fist/hoof Multiattack, against a target less than 15 feet away, does only 3d10 + 6d8 + 21 hp bludgeoning damage, or 64 hp on average. Even if it’s already engaged in melee, therefore, it would rather back up 20 feet (do you think a demon with AC 19 and 310 hp cares about opportunity attacks?) and then Charge again.
But let’s say that, like a dog chasing a thrown ball, the Charge is an instinctual reflex reaction to seeing an opponent at a certain distance. What if a goristro charges an opponent and fails to knock it down? Does it back up for another charge, or does it Multiattack with its fists and hooves?
I think this all depends on its relative likelihood of landing a hit, something the goristro wouldn’t be able to calculate but would have an instinctual sense of. A Charge attack is all-or-nothing: the goristro gets just one attack roll, and if it fails, no damage at all. Its fist and hoof attacks have the same probability to hit, but it does get three chances to land a hit (or miss). I’m going to say that if the goristro has a two-thirds or better chance of making its attack roll, it goes for the Charge, but if its chance to hit is lower than two-thirds, it hedges by making three attacks rather than one. With a to-hit modifier of +13, that means it must be attacking a target with AC 20 or greater (with disadvantage, AC 17 or greater; with advantage, AC 22 or greater) to prefer the fist/fist/hoof Multiattack.