The couatl is rarely encountered in Dungeons and Dragons campaigns, partly because of its distinctly Mesoamerican flavor (most campaign settings remain hardily quasi-European) and partly because of its lawful good alignment (good monsters make bad enemies), but the winged, feathered serpent has been part of the game since the first Advanced Dungeons and Dragons Monster Manual. In fifth edition, it’s categorized as a celestial.
Couatls have exceptional ability scores across the board, but their Dexterity and Wisdom are especially extraordinary. Their mental ability scores are higher on average than their physical ability scores, suggesting a preference for spellcasting over messy tooth-and-claw conflict (though their spell repertoire turns out not to support this preference especially well). Their 30-foot slithering speed is put to shame by their 90-foot flying speed; this, plus an awesome natural armor class of 19 and an immunity to physical damage from nonmagical weapons, implies that if they do engage in physical combat, they’ll most often hover in the air, dive-bomb their targets, then fly back out of reach without concern for opportunity attacks.
In addition to being immune to normal weapon damage, couatls are also immune to psychic damage and resistant to radiant damage. They have proficiency in two of the big three saving throws, Constitution and Wisdom, as well as Charisma, and magical mind-reading and scrying don’t work on them. They also have truesight with a range of 120 feet.
Strangely, even though Wisdom is their highest ability stat, Charisma is the ability they use for spellcasting—perhaps as a kindness to their opponents. Also, most of their spells are geared toward social interaction situations, not combat. This is especially true of their once-per-day spells, dream, greater restoration and scrying, all of which are favors that player characters might petition a couatl for. Similarly, detect evil and good, detect magic, detect thoughts, create food and water and lesser restoration are all spells without direct combat application.
From a tactical standpoint, the key spells to be aware of are bless, cure wounds, sanctuary and shield. Bless is a powerful support spell that a couatl will cast on its allies, if any are present, and sustain for the duration of combat. It will do this before it engages in any attack of its own, because lawful good, yo. Sanctuary is cast as a bonus action—important, since aside from this spell and shield, the couatl’s action economy is limited to one action per turn—and wards its target against attack, so a couatl will cast this on the most vulnerable creature under its protection.
Shield, cast as a reaction, is a simple self-defense measure that the couatl can cast up to three times, once per turn. With Intelligence 18 and Wisdom 20, the couatl always knows which incoming hit to cast shield against and which to let through.
Here’s the basic rule: cast shield against the opponent with the greatest expected damage (that is, average damage times probability to hit) from a spell attack, magical weapon, or nonmagical weapon attack that does some kind of damage other than bludgeoning, slashing and piercing (which can’t hurt it). If that opponent has already attacked and missed, cast it against No. 2; if No. 2 has attacked and missed, No. 3; and so on down the line. Actually, scratch all that. Since the shield spell lasts until the start of the couatl’s next turn, it can cast it at the first moment it needs to and enjoy the benefits for the remainder of its opponents’ attacks.
Cure wounds is weak for its action cost, consuming a full action to restore an average of only 8 hp. For this reason, a couatl probably won’t use it except to save a seriously wounded ally from imminent death, and in that case, it will combine it with a sanctuary spell, then spend the remainder of combat closely defending that ally.
OK, so even though it seems like a couatl should prefer spell attacks over melee attacks (couatls have no ranged attack), there’s really nothing in its repertoire that allows it to fight this way. Instead, it has two melee attack actions, Bite and Constrict. Bite does nominal damage but also has the potential to put an opponent to sleep. Constrict is the real power attack, because not only does it do more damage, it also grapples and restrains on a hit. What does this mean? First, that the couatl can attack a grappled target with advantage on a subsequent round. Second, the couatl can fly up into the air with its target and still do so with respectable speed, despite its movement’s being halved (see “Moving a Grappled Target,” Player’s Handbook, page 195).
Like other lawful good creatures, a couatl is unlikely to pick a fight, except with intrinsically evil beings such as fiends and undead creatures. However, some couatls are charged with guarding individuals, sacred places or holy relics, and if a fight is what it takes to do that, a fight is what you’ll get.
An attacking couatl will use its first turn to cast bless on any allies present if it needs to, along with sanctuary if one ally requires special protection. Otherwise, a couatl’s standard modus operandi is to hover in the air, out of reach of melee weapons, swoop down on the enemy who’s most ideologically opposed to it (see my post on angel tactics) and attack to constrict. If it misses, it flies back up out of reach; if it hits, it flies back up with its grappled target wrapped up in its coils. (Couatls don’t have the hover property attached to their flying speed, so they have to keep moving in order to use this tactic; they can’t just keep station in one place.)
On its next turn, a couatl with a grappled target in its coils bites it (with advantage, since the target is restrained), hoping to put it to sleep. If it succeeds, the next thing it does—if it can afford to leave its post—is to fly its opponent outside and dump him or her, like a celestial bouncer. Then it returns to see whether its remaining opponents are more open to reason.
This is a time-consuming tactic, though, so it’s only suitable for use against small groups (say, three-ish) or against medium-size groups (say, five-ish) with only a couple of determinedly evil and/or chaotic members. A couatl that has to fight off more than two or three enemies to accomplish its purpose has to forgo the benefits of attacking a restrained target and deal with its foes with more dispatch. In that case, it will rely on its bite attack instead, prioritizing its enemies by chaoticness and evilness and, one by one, swooping down and biting them until they pass out.
Couatls are creatures of duty, and they know when they’ll die, though not how. A couatl with a person, place or thing to protect will do so until death, but in the absence of such a duty, it will retreat to save itself when seriously wounded (reduced to 38 hp or fewer), flying at full speed while taking the Dodge action to avoid incoming attacks.